Hot-dip galvanized steel sheet

ABSTRACT

A hot-dip galvanized steel sheet includes a base steel sheet and a hot-dip galvanized layer formed on at least one surface of the base steel sheet, in which the hot-dip galvanized layer includes Fe in a content of more than 0% to 5% or less, Al in a content of more than 0% to 1.0% or less, and columnar grains formed by a ζ phase on the surface of the steel sheet, further, 20% or more of the entire interface between the hot-dip galvanized layer and the base steel sheet is coated with the ζ phase, and a ratio of an interface formed between grains in which coarse oxides are present among grains and the base steel sheet with respect to the entire interface between the ζ phase and the base steel sheet in the hot-dip galvanized layer is 50% or less, the base steel sheet has predetermined chemical components and a refined layer in direct contact with the interface between the base steel sheet and the hot-dip galvanized layer, an average thickness of the refined layer is 0.1 to 5.0 μm, an average grain size of ferrite in the refined layer is 0.1 to 3.0 μm, one or two or more of oxides of Si and Mn are contained in the refined layer, and a maximum size of the oxide is 0.01 to 0.4 μm, and a volume fraction of residual austenite in a range of ⅛ thickness to ⅜ thickness centered at a position of ¼ thickness from the surface of the base steel sheet is 1% or more.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a hot-dip galvanized steel sheetexcellent in plating adhesion.

Priority is claimed on Japanese Patent Application No. 2014-225398,filed on Nov. 5, 2014, the content of which is incorporated herein byreference.

RELATED ART

There has been an increasing demand for high-strengthening of steelsheets mainly used for automotive frame members. In order for these highstrength steel sheets to obtain high strength and excellent formability,an alloy element which is represented by Si or Mn and contributes to theimprovement of strength is generally added. However, the alloy elementwhich is represented by Si or Mn has an effect of deteriorating platingadhesion.

Since an automotive steel sheet is generally used outdoors, it isusually required for the steel sheet to have excellent corrosionresistance.

In uses for automotive outside sheets and the like, the peripheral partof a sheet is usually subjected to severe bending (hem bending) by pressworking. Not only in uses for the automotive outside sheets but also inother uses, a sheet is subjected to severe bending by press working,hole expansion working, or the like to be used in many cases. In thecase of subjecting a conventional hot-dip galvanized steel sheet tosevere bending, hole expansion working, or the like, the plated layer issometimes peeled off from the base steel sheet in the worked part. Whenthe plated layer is peeled off from the base steel sheet as describedabove, there is a problem that the corrosion resistance of the peeledpart is lost and the base steel sheet is corroded and rusted at an earlystage. In addition, even when the plated layer is not peeled off, theadhesion between the plated layer and the base steel sheet is lost, evensmall voids are formed in the area in which the adhesion is lost tocause external air or moisture to enter the voids, and thus a functionof corrosion resistance by the plated layer is lost. As a result, asdescribed above, corrosion and rusting occurs in the base steel sheet atan early stage.

In view of such problems, for a high strength steel sheet for uses inwhich such severe bending or the like is performed, there has been astrong desire for developing a plated steel sheet including a hot-dipgalvanized layer excellent in adhesion of the plated layer with the basesteel sheet.

In order to enhance the adhesion of a plated layer, for example, asrepresented by Patent Documents 1 to 3, methods of forming oxides insidea steel sheet and reducing the amount of oxides at an interface betweenthe base metal and a plated layer that causes plating peeling areproposed. However, in such a case of forming an oxide on the surface ofthe steel sheet, carbon in the surface of the steel sheet is bound tooxygen to be gasified. As a result, carbon is released from the steelsheet and thus the strength of the region from which the carbon isreleased is significantly decreased in some cases. In the case in whichthe strength of the surface of the steel sheet is decreased, there is aconcern that fatigue resistance, which strongly depends on theproperties of the surface part, is deteriorated and thus fatiguestrength is significantly decreased.

Alternatively, in order to enhance the adhesion of a plated layer, inPatent Document 4, a method of enhancing plating adhesion by reformingthe surface of a base steel sheet in such a manner that steps areperformed by adding new annealing step and pickling step before a normalannealing step, is proposed. However, in the method described in PatentDocument 4, the number of steps is increased as compared to a normalmethod of producing a high strength plated steel sheet, and thus thereis a problem in costs.

Further, in Patent Document 5, a method of enhancing plating adhesion byremoving carbon from the surface part of a base steel sheet is proposed.However, in the method described in Patent Document 5, the strength ofthe region from which carbon is removed is significantly decreased.Therefore, there is a concern that fatigue resistance, which stronglydepends on the properties of the surface part, is deteriorated and thusfatigue strength is significantly decreased in the method described inPatent Document 5.

In Patent Documents 6 and 7, there are disclosed steel sheets in whichthe amounts of Mn, Al, and Si in a plated layer are controlled to bewithin a suitable range and plating adhesion is improved. For the steelsheets described in Patent Documents 6 and 7, it is required to controlthe amounts of elements in the plated layer with high accuracy at thetime of production, which applies a great industrial load and causes aproblem in costs.

In Patent Document 8, a high strength steel sheet in which themicrostructure of the steel sheet is formed of only ferrite in order toenhance plating adhesion is proposed. However, since the microstructureis formed of only soft ferrite in the steel sheet described in PatentDocument 8, sufficiently high strength cannot be obtained.

Here, a galvannealed steel sheet obtained by subjecting a steel sheet toan alloying treatment after a hot dip galvanizing treatment is widelyused. The alloying treatment is a treatment of heating a plated layer toa temperature of equal to or higher than the melting point of Zn,diffusing a large amount of Fe atoms into the plated layer from theinside of a base steel sheet, and forming the plated layer into a layermainly including a Zn—Fe alloy. For example, in Patent Documents 9, 10and 11, galvannealed steel sheets excellent in plating adhesion areproposed. However, it is required to heat a steel sheet at a hightemperature so as to sufficiently alloy the plated layer. When the steelsheet is heated to a high temperature, the microstructure inside thesteel sheet is reformed and particularly coarse iron-based carbides areeasily generated and the properties of the steel sheet deteriorate.Thus, this case is not preferable.

In Patent Document 12, as a base steel sheet, a hot-dip galvanized steelsheet including one or more selected from the group consisting of Si, Mnand Al is disclosed. In Patent Document 12, the control of thetemperature at which the base steel sheet enters a plating bath in theproduction step is described. In addition, in Patent Document 12, ahot-dip galvanized steel sheet excellent in plating adhesion and spotweldability in which the area fraction of the cross section of the alloylayer formed at the interface between the base steel sheet and theplated layer is determined is disclosed.

In Patent Document 12, it is disclosed that when a steel sheet in whichSi and Mn oxides are present in the surface enters a hot dip galvanizingbath, a large amount of non-plating in which the steel sheet is notplated with zinc is generated. However, in Patent Document 12, atechnology for reducing the amounts of Si and Mn oxides until plating isstarted is not disclosed. In addition, in Patent Document 12, thetemperature at which the base steel sheet enters a plating bath is setto be higher than the temperature of the hot dip galvanizing bath.Although the temperature varies depending on the Al content in the hotdip galvanizing bath, the temperature at which the base steel sheetenters a plating bath is set to be at least 4° C. higher thantemperature of the hot dip galvanizing bath and to be at most 28° C.higher than temperature of the hot dip galvanizing bath. Therefore, inPatent Document 12, regarding the stability of the bath temperature,uniformity in the properties of the product is not sufficient in somecases.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Unexamined Patent Application, FirstPublication No. 2008-019465

[Patent Document 2] Japanese Unexamined Patent Application, FirstPublication No. 2005-060742

[Patent Document 3] Japanese Unexamined Patent Application, FirstPublication No. H9-176815

[Patent Document 4] Japanese Unexamined Patent Application, FirstPublication No. 2001-026853

[Patent Document 5] Japanese Unexamined Patent Application, First

[Patent Document 6] Japanese Unexamined Patent Application, FirstPublication No. 2003-055751

[Patent Document 7] Japanese Unexamined Patent Application, FirstPublication No. 2003-096541

[Patent Document 8] Japanese Unexamined Patent Application, FirstPublication No. 2005-200750

[Patent Document 9] Japanese Unexamined Patent Application, FirstPublication No. H11-140587

[Patent Document 10] Japanese Unexamined Patent Application, FirstPublication No. 2001-303226

[Patent Document 11] Japanese Unexamined Patent Application, FirstPublication No. 2005-060743

[Patent Document 12] Published Japanese Translation No. 2013-541645 ofthe PCT International Publication

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In consideration of the above circumstances, an object of the presentinvention is to provide a hot-dip galvanized steel sheet excellent instrength, ductility, hole expansibility, spot weldability and platingadhesion.

Means for Solving the Problem

The present inventors have conducted intensive investigations forobtaining a hot-dip galvanized steel sheet excellent in platingadhesion. As a result, the present inventors have found that platingpeeling is suppressed by forming a ζ phase (FeZn₁₃) in a plated layerand incorporating a coarse oxide, which acts as a fracture origin, inthe inside thereof. By the above method, a hot-dip galvanized steelsheet excellent in plating adhesion can be obtained without subjectingthe plating layer to an alloying treatment.

The present invention has been completed based on the findings andembodiments thereof are as follows.

(1) A hot-dip galvanized steel sheet comprising: a base steel sheet; anda hot-dip galvanized layer formed on at least one surface of a basesteel sheet, in which the hot-dip galvanized layer includes Fe in acontent of more than 0% to 5% or less, Al in a content of more than 0%to 1.0% or less, and columnar grains formed by a ζ phase on a surface ofthe steel sheet, further, 20% or more of an entire interface between thehot-dip galvanized layer and the base steel sheet is coated with the ζphase, and a ratio of an interface formed between ζ grains in whichcoarse oxides are present among ζ grains and the base steel sheet withrespect to the entire interface between the ζ phase and the base steelsheet in the hot-dip galvanized layer is 50% or less,

the base steel sheet includes a chemical composition which satisfies, %by mass,

C: 0.040% to 0.400%,

Si: 0.05% to 2.50%,

Mn: 0.50% to 3.50%,

P: 0.0001% to 0.1000%,

S: 0.0001% to 0.0100%,

Al: 0.001% to 1.500%,

N: 0.0001% to 0.0100%,

O: 0.0001% to 0.0100%, and

Si+0.7Al≧0.30 (in the expression, element symbols represent the content(% by mass) of each element), with a remainder of Fe and unavoidableimpurities,

the base steel sheet has a refined layer in direct contact with theinterface between the base steel sheet and the hot-dip galvanized layer,an average thickness of the refined layer is 0.1 to 5.0 μm, an averagegrain size of ferrite in the refined layer is 0.1 to 3.0 μm, one or twoor more of oxides of Si and Mn are contained in the refined layer, and amaximum size of the oxide is 0.01 to 0.4 μm, and

a volume fraction of residual austenite in a range of ⅛ thickness to ⅜thickness centered at a position of ¼ thickness from the surface of thebase steel sheet is 1% or more.

(2) The hot-dip galvanized steel sheet according to (1),

in which a plated amount on one surface of the base steel sheet in thehot-dip galvanized layer is 10 g/m² or more and 100 g/m² or less.

(3) The hot-dip galvanized steel sheet according to (1) or (2),

in which the base steel sheet further contains, % by mass, one or two ormore selected from

Ti: 0.001% to 0.150%,

Nb: 0.001% to 0.100%, and

V: 0.001% to 0.300%.

(4) The hot-dip galvanized steel sheet according to any one of (1) to(3),

in which the base steel sheet further contains, % by mass, one or two ormore selected from

Cr: 0.01% to 2.00%,

Ni: 0.01% to 2.00%,

Cu: 0.01% to 2.00%,

Mo: 0.01% to 2.00%,

B: 0.0001% to 0.0100%, and

W: 0.01% to 2.00%.

(5) The hot-dip galvanized steel sheet according to any one of (1) to(4),

in which the base steel sheet further contains, % by mass, one or two ormore selected from Ca, Ce, Mg, Zr, La, and REM in a total amount of0.0001% to 0.0100%.

EFFECTS OF THE INVENTION

According to the embodiment of the present invention, it is possible toprovide a hot-dip galvanized steel sheet excellent in strength,ductility, hole expansibility, spot weldability, and plating adhesion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron microscope (SEM) micrograph showing thecross section of the vicinity of an area including an interface betweena hot-dip galvanized layer and a base steel sheet in the cross sectionof a hot-dip galvanized steel sheet according to an embodiment.

FIG. 2 is a field emission scanning electron microscope (FE-SEM)micrograph showing a cross section of a hot-dip galvanized steel sheetaccording to the embodiment.

EMBODIMENTS OF THE INVENTION

A hot-dip galvanized steel sheet according to the present inventionincludes a base steel sheet (hereinafter, also referred to as a steelsheet simply) and a hot-dip galvanized layer formed on at least onesurface of the steel sheet (hereinafter, also referred to as a platedlayer simply).

The plated layer has a Fe content of more than 0% to 5% or less and anAl content of more than 0% to 1.0% or less, and includes columnar grainsformed of a ζ phase. In the plated layer, 20% or more of the entireinterface between the plated layer and the base steel sheet is coveredwith the ζ phase, the ratio of the interface formed between the ζ grainand the base steel sheet where coarse oxides are present in theinterface between the ζ phase and the base steel sheet is 50% or less.

First, the zinc-plated layer constituting the hot-dip galvanized steelsheet according to the embodiment of the embodiment of the presentinvention will be described. The term “%” in the following descriptionmeans “% by mass”.

(Plated Layer)

In the embodiment of the present invention, the hot-dip galvanized layerhas a Fe content of more than 0% to 5.0% or less and an Al content ofmore than 0% to 1.0% or less. Further, the hot-dip galvanized layer maycontain one or two or more of Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Ge,Hf, I, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, Pb, Rb, Sb, Si, Sn, Sr, Ta,Ti, V, W, Zr, and REM or one or two or more of these elements may bemixed in the hot-dip galvanized layer. Even when the hot-dip galvanizedlayer contains one or two or more of these elements or one or two ormore of these elements are mixed in the hot-dip galvanized layer asdescribed above, the effects of the present invention are notdeteriorated and there is sometimes a preferable case in which thecorrosion resistance and the workability are improved depending on thecontent of the element.

In addition, in the embodiment of the present invention, the hot-dipgalvanized layer includes columnar grains formed of a ζ phase and 20% ormore of the entire interface between the plated layer and the base steelsheet is covered with the ζ phase.

Further, it is preferable that the plated amount of the hot-dipgalvanized layer on one surface of the base steel sheet is 10 g/m² ormore and 100 g/m² or less.

[Fe Content in Hot-Dip Galvanized Layer: More Than 0% to 5.0% or Less]

When the Fe content in the hot-dip galvanized layer becomes higher, theplating adhesion is deteriorated and thus it is required that the Fecontent is 5.0% or less. In order to further enhance the platingadhesion, the Fe content in the plated layer is preferably 4.0% or lessand more preferably 3% or less. The lower limit of the Fe content in theplated layer is more than 0%. When the Fe content is less than 0.5%, theamount of ζ phase required to enhance adhesion is not sufficientlyobtained in some cases. Therefore, the Fe content in the plated layer ispreferably 0.5% or more and more preferably 1.0% or more.

[Al Content in Hot-Dip Galvanized Layer: More Than 0% to 1.0% or Less]

When the Al content in the hot-dip galvanized layer becomes higher, theplating adhesion is deteriorated and thus it is required that the Alcontent is 1.0% or less. In order to further enhance the platingadhesion, the Al content in the plated layer is preferably 0.8% or lessand more preferably 0.5% or less. The lower limit of the Al content inthe plated layer is more than 0%. In order to set the Al content to lessthan 0.01%, it is required that the concentration of Al in a platingbath is lowered extremely. When the concentration of Al in a platingbath is lowered extremely, the alloying of the plated layer excessivelyproceeds and thus the Fe content in the plated layer is increased sothat the plating adhesion is deteriorated. For this reason, the Alcontent in the plated layer is preferably 0.01% or more. From thisviewpoint, the Al content in the plated layer is more preferably 0.05%or more.

Furthermore, the hot-dip galvanized layer may contain one or two or moreof Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Ge, Hf, I, K, La, Li, Mg, Mn,Mo, Na, Nb, Ni, Pb, Rb, Sb, Si, Sn, Sr, Ta, Ti, V, W, Zr, and REM or oneor two or more of these elements are mixed in the hot-dip galvanizedlayer. Even when the hot-dip galvanized layer contains one or two ormore of these elements or one or two or more of these elements are mixedin the hot-dip galvanized layer as described above, the effects of thepresent invention are not deteriorated and there is sometimes apreferable case in which the corrosion resistance and the workabilityare improved depending on the content of the element.

[ζ N Phase]

FIG. 1 is a scanning electron microscope (SEM) micrograph showing thecross section of the hot-dip galvanized steel sheet according to theembodiment. As shown in FIG. 1, the hot-dip galvanized layer of thepresent invention includes columnar grains formed of a ζ phase(FeZn_(i3)) of an alloy of Fe and Zn. Particularly, the ratio of theinterface of the ζ phase in contact with the base steel sheet is 20% ormore in the entire interface between the plated layer and the base steelsheet. Accordingly, coarse oxides including Si and/or Mn, which acts asthe origin of peeling, and having a major axis of 0.2 μm or more areincorporated into the ζ phase from the surface of the base steel sheet.This makes the coarse oxides hardly work as a fracture origin and thusthe adhesion of the plated layer is improved. From this viewpoint, theratio of the interface between the ζ phase and the base steel sheet withrespect to the entire interface between the plated layer and the basesteel sheet is preferably 25% or more and more preferably 30% or more.The upper limit of the ratio of the interface between the ζ phase andthe base steel sheet with respect to the entire interface between theplated layer and the base steel sheet is not particularly limited andmay be 100%. When the major axis of the oxides including Si and /or Mnis 0.2 μm or more, cracking started from the oxides becomes remarkableand when major axis of the oxides is less than 0.2 μm, the oxides hardlywork as an origin of cracking. This is because a degree of stressconcentration varies depending on the size of the oxide at the time ofdeformation of the hot-dip galvanized steel sheet. Specifically, as thesize of the oxides increases (the major axis becomes longer), stress ismore easily concentrated at the time of deformation and the plated layeris more easily peeled off.

On the other hand, when the coarse oxides including Si and/or Mn is notincorporated into the ζ phase and the coarse oxides are present at theinterface between the ζ phase and the base steel sheet, the effect ofimproving plating adhesion by the ζ phase cannot be sufficientlyobtained and is not preferable. For this reason, the ratio of theinterface between ζ grains (coarse oxide-containing ζ grains) in whichcoarse oxides are present among the crystals of ζ phase grains) and thebase steel sheet is 50% or less with respect to the entire interfacebetween the ζ phase and the base steel sheet. In the case in which theratio of the interface between the coarse oxide-containing ζ grain andthe base steel sheet is 50% or less, the amount of the oxides includingSi and/or Mn, which is not incorporated into the ζ phase and is presenton the surface is sufficiently reduced. The ratio of the interfacebetween the coarse oxide-containing ζ grain and the base steel sheet ispreferably 35% or less with respect to the entire interface between theζ phase and the base steel sheet. The smaller amount of the coarseoxides having a major axis of 0.2 μm or more at the interface betweenthe ζ phase and the base steel sheet, the more preferable. In the entireinterface between the ζ phase and the base steel sheet, the ratio of theinterface formed between the coarse oxide-containing ζ grain and thebase steel sheet is most preferably 0%.

In addition, the hot-dip galvanized layer may include a δ1 phase(FeZn₇). However, in order to increase the fraction of the δ1 phase, thebase steel sheet is required to be heated to alloy the plated layerafter the base steel sheet is immersed in the plating bath, and thetensile properties of the base steel sheet are deteriorated due toheating. From this view point, it is preferable that the fraction of theδ1 phase is small. Particularly, the ratio of the interface of the δ1phase in contact with the base steel sheet is preferably 20% or less inthe entire interface between the plated layer and the base steel sheet.

In the present invention, the ratio of the interface between the ζ phaseand the base steel sheet with respect to the entire interface betweenthe plated layer and the base steel sheet and the ratio of the interfacebetween the δ1 phase and the base steel sheet with respect to the entireinterface between the plated layer and the base steel sheet can beobtained as follows.

That is, a thickness cross section parallel to the rolling direction ofthe base steel sheet is set as an observed section and a sample iscollected from the hot-dip galvanized steel sheet. The observed sectionis subjected to mirror polishing and observation is performed until thetotal length L of the observed interfaces between the plated layer andthe base steel sheet reaches 200 μm or more by using a field emissionscanning electron microscope (FE-SEM). When observation is performeduntil the total length L of the interfaces reaches 200 μm or more,observation may be performed in one thickness cross section until Lreaches 200 μm or more or observation may be performed in a plurality ofthickness cross sections until L reaches 200 μm or more.

In the same visual field as the visual field in which L is observed,grains having a columnar shape have the ζ phase or the δ1 phase and thetotal length L1 of the interfaces between the ζ phase and the δ1 phaseand the base steel sheet is measured. Subsequently, in the same visualfield as the visual field in which L1 is observed, high resolutioncrystal orientation analysis is performed according to EBSD (ElectronBach-scattering Diffraction) method using a FE-SEM to identify the δ1phase. Thus, the total length L2 of the interfaces between the δ1 phaseand the base steel sheet is obtained.

(L1-L2)/L is considered as the ratio of the interface between the ζphase and the base steel sheet in the entire interface between theplated layer and the base steel sheet.

In the same manner, L2/L is considered as the ratio of the interfacebetween the δ1 phase and the base steel sheet in the entire interfacebetween the plated layer and the base steel sheet.

The ζ phase and the δ1 phase may be identified according to methodsother than the above EBSD method. For example, the ζ phase and the δ1phase may be determined based on a difference in amount of Zn by mappingthe Zn element in the plated layer using a field emission electron probemicroanalyser (FE-EPMA).

In addition, the ratio of the interface between the grains in whichcoarse oxides are present (coarse oxide-containing ζ grains) among thecrystals of ζ (ζ grains) and the base steel sheet is obtained asfollows. That is, in the same visual field as the visual field in whichL is observed, the interface between the ζ phase and the base steelsheet is observed and ζ grains in which coarse oxides having a majoraxis of 0.2 μm or more are present (coarse oxide-containing ζ grains) isobtained at the interface between the ζ phase and the base steel sheet.The oxides present at the interface between the ζ phase of the platedlayer and the base steel sheet appear to be darker than the surroundingsin a SEM backscattered electron (BSE) image. Therefore, whether or notoxides are present at the interface between the ζ phase of the platedlayer and the base steel sheet is determined by observing a SEMbackscattered electron (BSE) image in the same visual field as thevisual field in which L is observed based on a difference in brightnessbetween the oxides and the surroundings. In addition, the major axis ofeach of the determined oxides on the observed section is measured andoxides having a major axis of 0.2 μm or more are determined as coarseoxides. Then, the length of the interface between the coarseoxide-containing grains and the base steel sheet is obtained and thetotal length L** of the interfaces thereof is obtained. The ratio of theinterface formed between the coarse oxide-containing ζ grains and thebase steel sheet in the entire interface between the ζ phase and thebase steel sheet is calculated by obtaining L**/(L1−L2).

In order to make the ζ phase appear to be clear, the observed section ofthe sample may be subjected to mirror polishing and then the observedsection may be corroded using a corrosive liquid such as nital.

[Plated Amount of Hot-Dip Galvanizing: 10 to 100 g/m²]

When the plated amount of the hot-dip galvanized layer on one surface ofthe base steel sheet is small, there is a concern that sufficientcorrosion resistance may not be obtained. For this reason, the platedamount of the plated layer on one surface of the base steel sheet ispreferably 10 g/m² or more. From the viewpoint of corrosion resistance,the plated amount is more preferably 20 g/m² or more and even morepreferably 30 g/m² or more. On the other hand, when the plated amount ofthe plated layer is large, the wear of electrodes is significant at thetime of performing spot welding and there is a concern of causingreduction in a weld nugget diameter or deterioration in welded jointstrength at the time of continuously performing spot welding. Therefore,the plated amount of the plated layer is preferably 100 g/m² or less.From the viewpoint of continuous weldability, the plated amount is morepreferably 93 g/m² or less and even more preferably 85 g/m² or less.

The hot-dip galvanized steel sheet of the present invention includes theplated layer and the base steel sheet has a refined layer shown below.

The refined later is a region in which the average grain size of ferritepresent in the outermost layer is ½ or less of the average grain size ofthe ferrite in the lower layer thereof. The boundary at which theaverage grain size of the ferrite in the refined layer is greater than ½of the average grain size of the ferrite in the lower layer thereof isdefined as a boundary between the refined later and the lower layerthereof.

The refined layer is in direct contact with the interface between thebase steel sheet and the hot-dip galvanized layer. The average thicknessof the refined layer is 0.1 to 5.0 μm. The average grain size of theferrite in the refined layer is 0.1 to 3.0 μm. The refined layercontains one or two or more of oxides of Si and Mn and the maximum sizeof the oxide is 0.01 to 0.4 μm.

When the average thickness of the refined layer is 0.1 μm or more, crackgeneration or extension is suppressed at the time of working the hot-dipgalvanized steel sheet. Therefore, the average thickness of the refinedlayer is 0.1 μm or more and preferably 1.0 μm. In addition, a refinedlayer having an average thickness of 5.0 μm or less can be formed whilesuppressing excessive alloying in a plating bath. Accordingly, it ispossible to prevent deterioration in plating adhesion caused by anexcessive Fe content in the plated layer. For this reason, the averagethickness of the refined layer is 5.0 μm or less and preferably 3.0 μmor less.

When the average grain size of the ferrite in the refined layer 0.1 μmor more, crack generation or extension is suppressed at the time ofworking the hot-dip galvanized steel sheet. Therefore, the average grainsize of the ferrite in the refined layer is 0.1 μm or more andpreferably 1.0 μm or more. In addition, when the average grain size ofthe ferrite in the refined layer is greater than 3.0 μm, the effect ofsuppressing crack generation or extension is limitative. Therefore, theaverage grain size of the ferrite in the refined layer is 3.0 μm or lessand preferably 2.0 μm or less.

Examples of one or two or more of oxides of Si and Mn contained in therefined layer include one or two or more selected from SiO₂, Mn₂SiO₄,MnSiO₃, Fe₂SiO₄, FeSiO₃, and MnO.

When the maximum size of one or two or more of oxides of Si and Mncontained in the refined layer is 0.01 μm or more, the plated layer inwhich the formation of a ζ phase sufficiently proceeds with theformation of a refined layer can be formed. The maximum size of theoxide is preferably 0.05 μm or more. In addition, the refined layer inwhich the maximum size of the oxide is 0.4 μm or less can be formedwhile suppressing excessive alloying of the plated layer. The maximumsize of the oxide is preferably 0.2 μm or less.

The average thickness of the refined layer and the average grain size ofthe ferrite in the refined layer are measured according to the methoddescribed below. A thickness cross section parallel to the rollingdirection of the base steel sheet is set as an observed section and asample is collected from the hot-dip galvanized steel sheet. Theobserved section of the sample is processed by using CP (Cross sectionpolisher) and a backscattered electron image is observed at amagnification of 5,000 with FE-SEM (Field Emission Scanning ElectronMicroscopy) for measurement.

The maximum size of one or two or more of oxides of Si and Mn containedin the refined layer is measured according to the method describedbelow. A thickness cross section parallel to the rolling direction ofthe base steel sheet is set as an observed section and samples arecollected from the hot-dip galvanized steel sheet. The observed sectionof each sample is processed with FIB (Focused Ion Beam) to prepare thinfilm samples. Thereafter, each thin film sample is observed with FE-TEM(Field Emission Transmission Electron Microscopy) at a magnification of30,000. Each thin film sample is observed in five visual fields and themaximum size of the diameter of the oxide measured in the whole visualfield is set as the maximum size of the oxide in the thin film sample.

The chemical components (composition) of the base steel sheetconstituting the hot-dip galvanized steel sheet according to theembodiment will be described below. In the following description, theterm “%” means “% by mass”.

[C: 0.040% to 0.400%]

C is an element to be added to enhance the strength of the base steelsheet. However, when the C content is more than 0.400%, the spotweldability is deteriorated, which is not preferable. Thus, the Ccontent is 0.400% or less. From the viewpoint of spot weldability, the Ccontent is preferably 0.300% or less and more preferably 0.220% or less.On the other hand, when the C content is less than 0.040%, the strengthis deteriorated and thus it is difficult to secure sufficient maximumtensile strength. Thus, the C content is 0.040% or more. In order tofurther increase the strength, the C content is preferably 0.055% ormore and more preferably 0.070% or more.

[Si: 0.05% to 2.50%]

Si is an element that suppresses formation of iron-based carbides in thebase steel sheet and enhances strength and formability. However, Si isan element that makes steel brittle. When the Si content is more than2.50%, a trouble such as cracking of a cast slab or the like easilyoccurs. Therefore, the Si content is 2.50% or less. Further, Si formsoxides on the surface of the base steel sheet in an annealing step tosignificantly impair plating adhesion. From this viewpoint, the Sicontent is preferably 2.00% or less and more preferably 1.60% or less.On the other hand, when the Si content is less than 0.05%, in a platingstep for the hot-dip galvanized steel sheet, a large amount of coarseiron-based carbides is formed and strength and formability deteriorate.Therefore, the Si content is 0.05% or more. From the viewpoint ofsuppressing formation of iron-based carbides, the Si content ispreferably 0.10% or more and more preferably 0.25% or more.

[Mn: 0.50% to 3.50%]

Mn is added to increase the strength by increasing the hardenability ofthe base steel sheet. However, when the Mn content is more than 3.50%, acoarse Mn-concentrated part is generated in the thickness central partof the base steel sheet and embrittlement easily occurs. Thus, a troublesuch as cracking of a cast slab easily occurs. Therefore, the Mn contentis 3.50% or less. In addition, an increase in the Mn content results indeterioration of spot weldability. For this reason, the Mn content ispreferably 3.00% or less and more preferably 2.80% or less. On the otherhand, when the Mn content is less than 0.50%, a large amount of softstructure during cooling after annealing is formed and thus it isdifficult to secure a sufficiently high maximum tensile strength.Accordingly, the Mn content is 0.50% or more. In order to furtherenhance strength, the Mn content is preferably 0.80% or more and morepreferably 1.00% or more.

[P: 0.0001% to 0.1000%]

P is an element that makes steel brittle and when the P content is morethan 0.1000%, a trouble such as cracking of a cast slab easily occurs.Therefore, the P content is 0.1000% or less. In addition, P is anelement that causes embrittlement of a molten part by spot welding, andthe P content is preferably 0.0400% or less and more preferably 0.0200%or less in order to obtain sufficient welded joint strength. On theother hand, a P content of less than 0.0001% results in a greatlyincreased production cost. Thus, the P content is 0.0001% or more andpreferably 0.0010% or more.

[S: 0.0001% to 0.0100%]

S is an element that is bounded to Mn and forms coarse MnS andformability such as ductility, stretch flangeability and bendabilitydeteriorates. Therefore, the S content is 0.0100% or less. In addition,S is an element that deteriorates spot weldability. Therefore, the Scontent is preferably 0.0060% or less and more preferably 0.0035% orless. On the other hand, a S content of less than 0.0001% results in agreatly increased production cost. Therefore, the S content is 0.0001%or more, preferably 0.0005% or more, and more preferably 0.0010% ormore.

[Al: 0.001% to 1.500%]

Al is an element that makes steel brittle. When the Al content is morethan 1.500%, a trouble such as cracking of a cast slab easily occurs andthus the Al content is 1.500% or less. In addition, when the Al contentis increased, spot weldability is deteriorated and thus the Al contentis preferably 1.200% or less and more preferably 1.000% or less. On theother hand, even when the lower limit of the Al content is notparticularly limited, the effects of the present invention areexhibited. Al is an unavoidable impurity present in the raw material ina very small amount and an Al content of less than 0.001% results in agreatly increased production cost. Therefore, the Al content is 0.001%or more. In addition, Al is an element that that is effective as adeoxidation material but in order to obtain a sufficient deoxidationeffect, the Al content is preferably 0.010% or more.

[N: 0.0001% to 0.0100%]

Since N is an element that forms a coarse nitride and deterioratesformability such as ductility, stretch flangeability and bendability,the amount of N added is preferably reduced. When the N content is morethan 0.0100%, deterioration in formability is significant and thus theupper limit of the N content is 0.0100%. In addition, an excessiveamount of N causes generation of blowholes at the time of welding andthe lower the content thereof is the better it is. From this viewpoint,the N content is preferably 0.0070% or less and more preferably 0.0050%or less. On the other hand, even when the lower limit of the N contentis not particularly limited, the effects of the present invention areexhibited. However, an N content of less than 0.0001% results in agreatly increased production cost. Therefore, the lower limit of the Ncontent is 0.0001% or more. The N content is preferably 0.0003% or moreand more preferably 0.0005% or more.

[O: 0.0001% to 0.0100%]

Since O forms an oxide and deteriorates formability such as ductility,stretch flangeability and bendability, the content thereof is preferablyreduced. When the O content is more than 0.0100%, deterioration informability is significant and thus the upper limit of the upper limitof the 0 content is 0.0100%. Further, the O content is preferably0.0050% or less and more preferably 0.0030% or less. Even when the lowerlimit of the O content is not particularly limited, the effects of thepresent invention are exhibited. However, an O content of less than0.0001% results in a greatly increased production cost. Therefore, thelower limit thereof is 0.0001%. The O content is preferably 0.0003% ormore and more preferably 0.0005% or more.

[Si+0.7Al≧0.30]

Si and Al are elements that suppress formation of carbide with bainitictransformation. In order to obtain residual austenite, it is preferableto add a predetermined amount or more of Si and/or Al. From thisviewpoint, it is required that the amount of Si added and the amount ofAl added satisfy the following Expression 2. The value of the left side(Si+0.7Al) of the following Expression 2 is preferably 0.45 or more andmore preferably 0.70 or more.

Si+0.7Al≧0.30   (Expression 2)

In Expression 2, each of Si and Al represents the amount [% by mass] ofeach element added.

Further, the following elements may be optionally added to the basesteel sheet of the hot-dip galvanized steel sheet according to theembodiment of the present invention.

Specifically, the base steel sheet may contain one or two or moreselected from Ti: 0.001% to 0.150%, Nb: 0.001% to 0.100%, and V: 0.001%to 0.300%, in addition to the above chemical components.

[Ti: 0.001% to 0.150%]

Ti is an element that contributes to increasing the strength of the basesteel sheet by precipitate strengthening, fine grain strengthening dueto suppression of ferrite grain growth, and dislocation strengtheningthrough suppression of recrystallization. However, when the Ti contentis more than 0.150%, the amount of precipitated carbonitrides isincreased formability deteriorates. Thus, the Ti content is preferably0.150% or less. In addition, from the viewpoint of formability, the Ticontent is more preferably 0.080% or less. On the other hand, even whenthe lower limit of the Ti content is not particularly limited, theeffects of the present invention are exhibited. In order to sufficientlyobtain the effect of high-strengthening by adding Ti, the Ti content ispreferably 0.001% or more. In order to achieve higher strength of thebase steel sheet, the Ti content is more preferably 0.010% or more.

[Nb: 0.001% to 0.100%]

Nb is an element that contributes to increasing the strength of the basesteel sheet by precipitate strengthening, fine grain strengthening dueto suppression of ferrite grain growth, and dislocation strengtheningthrough suppression of recrystallization. However, when the Nb contentis more than 0.100%, the amount of precipitated carbonitrides isincreased and formability deteriorates. Thus, the Nb content is morepreferably 0.100% or less. From the viewpoint of formability, the Nbcontent is more preferably 0.060% or less. On the other hand, even whenthe lower limit of Nb content is not particularly limited, the effectsof the present invention are exhibited. In order to obtain asufficiently obtain the effect of high-strengthening by adding Nb, theNb content is preferably 0.001% or more. In order to achieve higherstrength of the base steel sheet, the Nb content is more preferably0.005% or more.

[V: 0.001% to 0.300%]

V is an element that contributes to increasing the strength of the basesteel sheet by precipitate strengthening, fine grain strengthening dueto suppression of ferrite grain growth, and dislocation strengtheningthrough suppression of recrystallization. However, when the V content ismore than 0.300%, the amount of precipitated carbonitrides is increasedand formability deteriorates. Therefore, the V content is preferably0.300% or less and more preferably 0.200% or less. On the other hand,even when the lower limit of the V content is not particularly limited,the effects of the present invention are exhibited. In order tosufficiently obtain the effect of high-strengthening by adding V, the Vcontent is preferably 0.001% or more and more preferably 0.010% or more.

In addition, the base steel sheet according to the embodiment maycontain one or two or more selected from Cr: 0.01 to 2.00%, Ni: 0.01% to2.00%, Cu: 0.01% to 2.00%, Mo: 0.01% to 2.00%, B: 0.0001% to 0.0100%,and W: 0.01% to 2.00%.

[Cr: 0.01% to 2.00%]

Cr is an element that suppresses phase transformation at a hightemperature and is effective for high-strengthening and may be addedinstead of part of C and/or Mn. However, when the Cr content is morethan 2.00%, hot workability is impaired and productivity deteriorates.Thus, the Cr content is preferably 2.00% or less and more preferably1.20% or less. On the other hand, even when the lower limit of the Crcontent is not particularly limited, the effects of the presentinvention are exhibited. However, in order to sufficiently obtain theeffect of high-strengthening by adding Cr, the Cr content is preferably0.01% or more and more preferably 0.10% or more.

[Ni: 0.01% to 2.00%]

Ni is an element that suppresses phase transformation at a hightemperature and is effective for high-strengthening and may be addedinstead of part of C and/or Mn. However, when a Ni content is more than2.00%, weldability is impaired. Thus, the Ni content is preferably 2.00%or less and more preferably 1.20% or less. On the other hand, even whenthe lower limit of the Ni content is not particularly limited, theeffects of the present invention are exhibited. In order to sufficientlyobtain the effect of high-strengthening by adding Ni, the Ni content ispreferably 0.01% or more and more preferably 0.10% or more.

[Cu: 0.01% to 2.00%]

Cu is an element that that exists as fine particles in steel to therebyenhance strength and can be added instead of part of C and/or Mn.However, when the Cu content is more than 2.00%, weldability isimpaired. Thus, the Cu content is preferably 2.00% or less and morepreferably 1.20% or less. On the other hand, even when the lower limitof the Cu content is not particularly limited, the effects of thepresent invention are exhibited. In order to sufficiently obtain theeffect of high-strengthening by adding Cu, the Cu content is preferably0.01% or more and more preferably 0.10% or more.

[Mo: 0.01% to 2.00%]

Mo is an element that suppresses phase transformation at a hightemperature and is effective for high-strengthening and may be addedinstead of part of C and/or Mn. However, when the Mo content is morethan 2.00%, hot workability is impaired and productivity deteriorates.Thus, the Mo content is preferably 2.00% or less and more preferably1.20% or less. On the other hand, even when the lower limit of the Mocontent is not particularly limited, the effects of the presentinvention are exhibited. In order to sufficiently obtain the effect ofhigh-strengthening by adding Mo, the Mo content is preferably 0.01% ormore and more preferably 0.05% or more.

[B: 0.0001% to 0.0100%]

B is an element that suppresses phase transformation at a hightemperature and is effective for high-strengthening and may be addedinstead of part of C and/or Mn. However, when the B content is more than0.0100%, hot workability is impaired and productivity deteriorates.Thus, the B content is preferably 0.0100% or less. From the viewpoint ofproductivity, the B content is more preferably 0.0050% or less. On theother hand, even when the lower limit of the B content is notparticularly limited, the effects of the present invention areexhibited. In order to sufficiently obtain the effect ofhigh-strengthening by adding B, the B content is preferably 0.0001% ormore. In order to achieve further high-strengthening, the B content ismore preferably 0.0005% or more.

[W: 0.01% to 2.00%]

W is an element that suppresses phase transformation at a hightemperature and is effective for high-strengthening and may be addedinstead of part of C and/or Mn. However, when the W content is more than2.00%, hot workability is impaired and productivity deteriorates. Thus,the W content is preferably 2.00% or less and more preferably 1.20% orless. On the other hand, even when the lower limit of the W content isnot particularly limited, the effects of the present invention areexhibited. In order to sufficiently obtain the effect ofhigh-strengthening by adding W, the W content is preferably 0.01% ormore and more preferably 0.10% or more.

The base steel sheet in the hot-dip galvanized steel sheet according tothe embodiment of the present invention may further contain, as anotherelements, one or two or more of Ca, Ce, Mg, Zr, La, and REM in a totalamount of 0.0001% to 0.0100%. The reasons for adding these elements areas follows.

Note that REM stands for Rare Earth Metal and refers to an elementbelonging to the lanthanoid series. In this embodiment, REM or Ce isoften added in misch metal and may contain elements of the lanthanoidseries other than La and Ce in a complex form. The effects of thepresent invention are exhibited even when elements of the lanthanoidseries other than La and Ce are contained in the slab as inevitableimpurities. Further, the effects of the present invention are exhibitedeven when metals La and Ce are added to the slab.

Ca, Ce, Mg, Zr, La, and REM are elements effective for improvingformability, and one or two or more of these elements can be added tothe slab. However, when the total content of one or two or more of Ca,Ce, Mg, Zr, La, and REM is more than 0.0100%, there is a concern ofductility being impaired. Therefore, the total content of the respectiveelements is preferably 0.0100% or less and more preferably 0.0070% orless. On the other hand, even when the lower limit of the content of oneor two or more of Ca, Ce, Mg, Zr, La, and REM is not particularlylimited, the effects of the present invention are exhibited. In order tosufficiently obtaining the effect of improving the formability of thebase steel sheet, the total content of one or two or more of therespective elements is preferably 0.0001% or more. From the viewpoint offormability, the total content of one or two or more of Ca, Ce, Mg, Zr,La, and REM is more preferably 0.0010% or more.

In the chemical components of the base steel sheet of the plated steelsheet according to the embodiment, a remainder other than theabove-described respective elements includes Fe and unavoidableimpurities. Incidentally, a very small amount of each of Ti, Nb, V, Cr,Ni, Cu, Mo, B, and W described above being less than the above-describedlower limit value is allowed to be contained as an impurity. Inaddition, regarding Ca, Ce, Mg, Zr, La, and REM, a minute amount of thembeing less than the above-described lower limit value of the totalcontent of them is allowed to be contained as an impurity.

(Microstructure)

Next, the microstructure of the base steel sheet of the hot-dipgalvanized steel sheet according to the embodiment of the presentinvention will be described.

[Residual Austenite]

The base steel sheet of the hot-dip galvanized steel sheet according tothe embodiment of the present invention includes residual austenite. Theresidual austenite is a structure for greatly improving a balancebetween strength and ductility. When the volume fraction of the residualaustenite within a range of ⅛ thickness to ⅜ thickness of which thecenter is at the position of ¼ thickness from the surface of the basesteel sheet is less than 1%, the effect of improving a balance betweenstrength and ductility is weak. Therefore, the volume fraction of theresidual austenite is 1% or more. In order to improve a balance betweenstrength and ductility, the volume fraction of the residual austenite ispreferably 3% or more and more preferably 5% or more. On the other hand,in order to obtain a large amount of residual austenite, it is requiredto significantly increase the amount of C to be added and as a result,there is a concern of deterioration in weldability due to a large amountof C. Therefore, the volume fraction of the residual austenite ispreferably 25% or less. In addition, the residual austenite istransformed into hard martensite with deformation and this martensiteworks as a fracture origin so that stretch flangeability isdeteriorated. Thus, the volume fraction of the residual austenite ismore preferably 20% or less.

The base steel sheet of the hot-dip galvanized steel sheet according tothe embodiment of the present invention may have a microstructure formedof one or two or more of granular ferrite, needle-like ferrite,unrecrystallized ferrite, pearlite, bainite, bainitic ferrite,martensite, tempered martensite, and coarse cementite, in addition tothe residual austenite. For the base steel sheet, the details of thevolume fraction of each phase and each structure, structure size, andarrangement can be appropriately selected to obtain properties accordingto the applications of the hot-dip galvanized steel sheet.

The volume fraction of each structure contained in the base steel sheetof the hot-dip galvanized steel sheet according to the embodiment of thepresent invention can be measured by the method, for example describedbelow.

The volume fraction of the residual austenite included in the structureof the base steel sheet of the hot-dip galvanized steel sheet accordingto the embodiment is evaluated according to an X-ray diffraction method.Within a range of ⅛ thickness to ⅜ thickness of the thickness from thesurface of the sheet, a surface parallel to the sheet surface is mirrorfinished and the area fraction of FCC iron is measured according to anX-ray diffraction method. The measured area fraction is used as thevolume fraction of the residual austenite.

The volume fraction of each of ferrite, bainitic ferrite, bainite,tempered martensite, fresh martensite, pearlite, and coarse cementiteincluded in the structure of the base steel sheet of the hot-dipgalvanized steel sheet according to the embodiment is calculatedaccording to the method described below. The thickness cross sectionparallel to the rolling direction of the base steel sheet is set as anobserved section and a sample is collected. The observed section of thesample is polished and etched with nital. The range of ⅛ thickness to ⅜thickness centered at the position of ¼ of the thickness from thesurface of the base steel sheet is observed with a field emissionscanning electron microscope (FE-SEM) to measure the area fractions ofthe structures and these area fractions can be considered as the volumefractions of the respective structures.

In addition, in the hot-dip galvanized steel sheet according to theembodiment, the thickness of the base steel sheet is not particularlylimited but from the viewpoint of flatness of the hot-dip galvanizedsteel sheet and controllability at the time of cooling, the thickness ofthe base steel sheet is suitably within a range of 0.6 mm or more andless than 5.0 mm.

(Method of Producing Hot-Dip Galvanized Steel Sheet)

Next, the method of producing the hot-dip galvanized steel sheetaccording to the embodiment will be described in detail.

The method of producing the plated steel sheet according to theembodiment includes an annealing step, a plating step, and a coolingstep after the plating step, and a bainitic transformation treatment forobtaining residual austenite, which will be described later, isperformed between the annealing step and the plating step and/or duringthe cooling step after the plating step.

The annealing step is a step of heating a base steel sheet to 750° C. orhigher at an average heating rate of 1.0° C./second in a range of 600°C. to 750° C. The plating step is a step of hot-dip galvanizing thesteel sheet surface by immersing the base steel sheet in a zinc platingbath under the conditions of a steel sheet temperature of 440° C. to480° C. and an amount of effective Al of 0.050% to 0.180% by mass in theplating bath when the steel sheet enters the plating bath having aplating bath temperature of 450° C. to 470° C., to form a plated layer.In the cooling step after the plating step, a cooling process of coolingthe steel sheet to 350° C. after the plating step satisfies thefollowing Expression (1), which will be described later.

In order to produce the hot-dip galvanized steel sheet according to theembodiment of the present invention, first, a base steel sheet isproduced.

The base steel sheet is produced by casting a slab to which alloyelements are added according to the properties, hot-rolling the slab,and cold-rolling the slab.

Hereinafter, each production step will be described in detail.

[Casting Step]

First, a slab to be supplied to hot rolling is cast. The chemicalcomponents (composition) of the slab are preferably the above-describedcomponents. For the slab to be supplied to hot rolling, a continuouscasting slab or a slab produced by a thin slab caster or the like can beused.

[Hot Rolling Step]

In the hot rolling step, in order to suppress anisotropy of crystalorientation caused by casting, the heating temperature of the slab ispreferably 1,080° C. or higher. The heating temperature of the slab ismore preferably 1,150° C. or higher. On the other hand, the upper limitof the heating temperature of the slab is not particularly limited. Inorder to heat the slab at higher than 1,300° C., a large amount ofenergy needs to be applied, which causes a significant increase in theproduction cost. Thus, the heating temperature of the slab is preferably1,300° C. or lower.

After heating the slab, hot rolling is performed. When the temperaturewhen the hot rolling is completed (rolling completion temperature) islower than 850° C., the rolling reaction force is high and thus it isdifficult to stably obtain a predetermined thickness. Therefore, thetemperature when the hot rolling is completed is preferably 850° C. orhigher and more preferably 870° C. or higher. On the other hand, inorder to set the temperature when the hot rolling is completed to behigher than 980° C., in the step from the completion of heating of theslab to the completion of hot rolling, a device for heating the steelsheet is necessary and a high cost is required. Therefore, thetemperature when the hot rolling is completed is preferably 980° C. orlower and more preferably 960° C. or lower.

Next, the hot-rolled steel sheet which has been subjected to hot rollingis coiled as a coil. The average cooling rate in the cooling processfrom the hot rolling to the coiling is preferably 10° C./second or more.This is because when transformation proceeds at a lower temperature, thegrain size of the hot-rolled steel sheet is made fine and the effectivegrain size of the base steel, sheet after cold rolling and annealing ismade fine.

The coiling temperature of the hot-rolled steel sheet is preferably 350°C. or higher and 750° C. or lower. This is because in the microstructureof the hot-rolled steel sheet, pearlite and/or coarse cementite having amajor axis of 1 μm or more is formed in a dispersed manner, strainintroduced by cold rolling is localized, and reverse transformation toaustenite having various crystal orientations occurs in the annealingstep. Thus, the effective crystal orientation of the base steel sheetafter annealing is refined. When the coiling temperature is lower than350° C., pearlite and/or coarse cementite may not be folined and thusthis case is not preferable. In addition, in order to easily performcold rolling by decreasing the strength of the hot-rolled steel sheet,the coiling temperature is more preferably increased to 450° C. orhigher. On the other hand, when the coiling temperature is higher than750° C., pearlite and ferrite are formed in a belt shape long in therolling direction, and effective grains of the base steel sheetgenerated from the ferrite part after cold rolling and annealing tend toextend in the rolling direction and be coarse, which is not preferable.In addition, in order to refine the grain size of the effective grainsof the base steel sheet after annealing, the coiling temperature is morepreferably decreased to 680° C. or lower.

Next, pickling of the hot-rolled steel sheet produced in theabove-described manner is performed. The pickling is performed forremoving oxides on the surface of the hot-rolled steel sheet. Thus, thepickling is important to improve plating adhesion of the base steelsheet. The pickling may be performed at once or a plurality of timesseparately.

[Cold Rolling Step]

Next, the hot-rolled steel sheet after pickling is subjected to coldrolling to obtain a cold-rolled steel sheet.

In the cold rolling, when the total rolling reduction is more than 85%,the ductility of the steel sheet is impaired and a risk of breaking ofthe steel sheet during the cold rolling becomes higher. Therefore, thetotal rolling reduction is preferably 85% or less. From this viewpoint,the total rolling reduction is more preferably 75% or less and even morepreferably 70% or less. The lower limit of the total rolling reductionin the cold rolling step is not particularly limited. When the totalrolling reduction is less than 0.05%, the shape of the base steel sheetis not uniform and plating adheres unevenly, so that an externalappearance of the steel sheet is impaired. Therefore, the total rollingreduction is preferably 0.05% or more and more preferably 0.10% or more.The cold rolling is preferably performed in a plurality of passes, butany number of passes of the cold rolling and any rolling reductiondistribution to each pass are applicable.

When the total rolling reduction in the cold rolling is within a rangeof more than 10% and less than 20%, recrystallization does not progresssufficiently in the following annealing step and coarse grains in whichmalleability is lost by including a large amount of dislocations remainnear the surface, and bendability and fatigue resistance properties maybe deteriorated in some cases. In order to avoid this, it is effectiveto make malleability remain by reducing the total rolling reduction andreducing accumulation of dislocations to the grains. Alternatively, itis also effective to turn the processed structure into recrystallizedgrains having a small amount of accumulation of dislocations inside byreducing the total rolling reduction and making recrystallizationsufficiently proceed in the annealing step. From the viewpoint ofreducing the accumulation of dislocations to the grains, the totalrolling reduction in the cold rolling is preferably 10% or less and morepreferably 5.0% or less. On the other hand, in order to makerecrystallization sufficiently proceed in the annealing step, the totalrolling reduction is preferably 20% or more and more preferably 30% ormore.

[Annealing Step]

In the embodiment of the present invention, the cold-rolled steel sheetis subjected to annealing. In the embodiment of the present invention, acontinuous annealing and plating line having a preheating zone, areduction zone, and a plating zone is preferably used. While performingthe annealing process, the steel sheet is allowed to pass though thepreheating zone and the reduction zone and before the steel sheetreaches the plating zone, the annealing step is completed. Then, theplating step is preferably performed in the plating zone.

As described above, in the case of using a continuous annealing andplating line in the annealing step and the plating step, for example,the method described below is preferably used.

The heating rate in the annealing step is related to the progress ofdecarburization in the steel sheet surface part through the treatmenttime in the preheating zone. When the heating rate in the annealing stepis low, the steel sheet is exposed to an oxidation atmosphere in thepreheating zone for a long period of time and thus decarburizationproceeds in the steel sheet surface part. In addition, when the heatingrate is too low, oxidation of the steel sheet proceeds and coarse oxidesare formed inside the steel sheet in some cases. Particularly, theheating rate at 600° C. to 750° C. is important and in order to avoidexcessive decarburization and oxidation in the steel sheet surface part,the average heating rate during the heating is 1.0° C./second or more.In order to avoid decarburization in the steel sheet surface, theaverage heating temperature at 600° C. to 750° C. is preferably 1.5°C./second or more and more preferably 2.0° C./second or more. Theaverage heating temperature at 600° C. to 750° C. is preferably 50°C./second or less to secure the treatment time in the preheating zone topromote ζ phase formation. When the average heating rate is 50°C./second or less, a plated layer in which the ratio of the interfacebetween the ζ phase and the base steel sheet in the entire interfacebetween the plated layer and the base steel sheet is larger is obtained.In order to sufficiently promote ζ phase formation, the average heatingrate is more preferably 10° C./second or less.

In the preheating zone, the steel sheet surface part is subjected to anoxidation treatment for forming a Fe oxide coating film having anappropriate thickness. At this time, the steel sheet is allowed to passthrough the preheating zone in which the air ratio in the mixed gas ofair and fuel gas used for a preheating burner, which will be describedbelow, is 0.7 or more, while heating the steel sheet to a steel sheettemperature of 400° C. to 800° C.

The term “air ratio” is a ratio between “the volume of air included inthe mixed gas per unit volume” and “the volume of air which istheoretically required to cause complete combustion of fuel gascontained in the mixed gas per unit volume”, and is represented by thefollowing expression.

Air ratio=[volume of air included in the mixed gas per unit volume(m³)]/[volume of air which is theoretically required to cause completecombustion of fuel gas contained in the mixed gas per unit volume (m³)]}

In the embodiment, the base steel sheet which is allowed to pass throughthe preheating zone is heated under the above conditions to form a Feoxide coating film (oxide) having a thickness of 0.01 to 5.0 μm on thesurface part of the base steel sheet. The Fe oxide coating film (oxide)formed on the steel sheet surface is reduced in the reduction zone andbecomes a surface excellent in plating adhesion.

In the case in which the air ratio is more than 1.2 and too high,excessive Fe oxide coating film is formed on the steel sheet surfacepart and after reduction, the decarburized layer becomes excessivelythick. Accordingly, the air ratio is preferably 1.2 or less and morepreferably 1.1 or less. In the case in which air ratio is less than 0.7and is too low, a predetermined oxide cannot be obtained. Thus, the airratio is 0.7 or more and preferably 0.8 or more.

When the steel sheet temperature for allowing the steel sheet to passthrough the preheating zone is lower than 400° C., a sufficient oxidefilm cannot be formed. Accordingly, the steel sheet temperature forallowing the steel sheet to pass through the preheating zone (preheatingcompletion temperature) is 400° C. or higher and preferably 600° C. orhigher. On the other hand, when the steel sheet temperature for allowingthe steel sheet to pass through the preheating zone is a hightemperature of higher than 800° C., reduction cannot be performed in thenext reduction zone and coarse oxides including Si and/or Mn are formedin the steel sheet surface part. Accordingly, the steel sheettemperature for allowing the steel sheet to pass through the preheatingzone is 800° C. or lower and preferably 750° C. or lower.

The maximum heating temperature in the annealing step is an importantfactor for controlling the fraction of the microstructure related to theformability of the steel sheet to be within a predetermined range. Whenthe maximum heating temperature is low, a large amount of coarseiron-based carbides is left unmelted in the steel and thus formabilityis deteriorated. In order to sufficiently solid-dissolve the iron-basedcarbides to enhance formability, the maximum heating temperature is 750°C. or higher. Particularly, in order to obtain residual austenite, it isrequired that the maximum heating temperature is (Ac1+50)° C. or higher.The upper limit of the maximum heating temperature is not particularlylimited but from the viewpoint of plating adhesion, the maximum heatingtemperature is preferably 950° C. or lower and more preferably 900° C.or lower for reducing oxides on the surface of the base steel sheet.

The Ac1 point of the steel sheet is a starting point of austenitereverse transformation. Specifically, the Acl point is obtained bycutting off a small piece from the steel sheet after hot rolling,heating the piece to 1,200° C. at 10° C./second, and measuring theamount of volume expansion during heating.

The temperature reaches the maximum heating temperature in the annealingstep (750° C. or higher) in the reduction zone. In the reduction zone,the thin Fe oxide coating film formed on the steel sheet surface in thepreheating zone is reduced to enhance plating adhesion. Therefore, aratio between a water vapor partial pressure P(H₂O) and a hydrogenpartial pressure P(H₂), P(H₂O)/P(H₂), in the atmosphere in the reductionzone is 0.0001 to 2.00. When P(H₂O)/P(H₂) is less than 0.0001, Si and/orMn oxides which act as a plating peeling origin are formed on theoutermost layer. On the other hand, when the P(H₂O)/P(H₂) is more than2.00, refinement excessively proceeds in the steel sheet surface andalloying of the plated layer excessively proceeds. Thus, platingadhesion is deteriorated. Further, when the P(H₂O)/P(H₂) is more than3.00, decarburization excessively proceeds and a hard phase of the basesteel sheet surface is remarkably reduced. From this viewpoint,P(H₂O)/P(H₂) is preferably within a range of 0.002 to 1.50 and morepreferably within a range of 0.005 to 1.20.

As described above, when P(H₂O)/P(H₂) in the reduction zone is 0.0001 to2.00, Si and/or Mn oxides which act as a plating peeling origin are notformed on the outermost layer and fine Si and/or Mn oxides having amaximum size of 0.01 to 0.4 gm are formed inside the steel sheetsurface. The fine Si and/or Mn oxides suppress the growth of Ferecrystallization during annealing. In addition, water vapor in thereduction atmosphere causes the base metal surface to be decarburizedand thus the base metal surface after annealing is turned into ferrite.As a result, on the surface of the base metal after annealing, a refinedlayer having an average thickness of 0.1 to 5.0 gm and having a ferritehaving an average grain size of 0.1 to 3.0 μm, and containing Si and/orMn oxides having a maximum size of 0.01 to 0.4 μm is formed.

In the annealing step, at a cooling step before the plating step afterthe temperature reaches the maximum heating temperature and before thesteel sheet reaches a plating bath (cooling step before plating), inorder to obtain residual austenite, it is required to suppress formationof pearlite and cementite. Therefore, in the cooling step beforeplating, the average cooling rate from 750° C. to 700° C. is 1.0°C./second or more and further the average cooling rate from 700° C. to500° C. is 5.0° C./second or more. Although the upper limit of theaverage cooling rate is not particularly provided, an excessively highaverage cooling rate is not preferable since a special cooling facilityand a coolant which does not interfere with the plating step arerequired to obtain the excessively high average cooling rate. From thisviewpoint, the average cooling rate in the above-described temperaturerange is preferably 100° C./second or less and more preferably 70°C./second or less.

Subsequent to the cooling step before plating, in order to obtaintempered martensite, in a period after the steel sheet temperaturereaches 500° C. and before the steel sheet reaches a plating bath, as amartensitic transformation treatment, the steel sheet may be retained ina predetermined temperature range for a predetermined period of time.Regarding the martensitic transformation treatment temperature, amartensitic transformation starting temperature Ms point is set as anupper limit and the upper limit is more preferably (Ms point −20° C.).The lower limit in the martensitic transformation treatment ispreferably 50° C. and the lower limit is more preferably 100° C. Inaddition, the martensitic transformation treatment time is preferably 1second to 100 seconds and more preferably 10 seconds to 60 seconds. Themartensite obtained in the martensitic transformation treatment enters aplating bath at a high temperature in the plating step and then ischanged into tempered martensite.

The Ms point is calculated by the following expression.

Ms Point [° C.]=541−474 C/(1−VF)−15Si−35Mn−17Cr−17Ni+19Al

In the above expression, VF represents the volume fraction of ferrite,and each of C, Si, Mn, Cr, Ni, and Al represents the amount [% by mass]of each element added.

It is difficult to directly measure the volume fraction of ferriteduring production. Therefore, when the Ms point is determined in thepresent invention, a small piece is cut off from the cold-rolled steelsheet before the steel sheet is allowed to pass through the continuousannealing and plating line. The small piece is annealed at the sametemperature as in the case in which the small piece is allowed to passthrough the continuous annealing and plating line and a change in thevolume of the ferrite of the small piece is measured so that a numericalvalue calculated using the result is used as the volume fraction VF ofthe ferrite.

Further, after the cooling step before plating, in order to obtainresidual austenite, as a bainitic transformation treatment, the steelsheet may be retained at a temperature range of 250° C. to 500° C. for apredetermined period of time.

The bainitic transformation treatment may be performed between theannealing step and the plating step, in the cooling step before plating,or in both steps.

It is required that the sum of the retaining periods of the bainitictransformation treatment that is performed between the annealing stepand the plating step and in the cooling step before plating is 15seconds or longer and 500 seconds or shorter. When the sum of theretaining periods is 15 seconds or longer, bainitic transformationsufficiently proceeds and a sufficient amount of residual austenite canbe obtained. The sum of the retaining periods is preferably 25 secondsor longer. On the other hand, when the sum of the retaining periods ismore than 500 seconds, pearlite and/or coarse cementite is formed.Therefore, the sum of the retaining periods is 500 seconds or shorterand preferably 300 seconds or shorter.

In the case in which the bainitic transformation treatment is performedbetween the annealing step and the plating step, when the bainitictransformation treatment temperature is higher than 500° C., pearliteand/or coarse cementite is formed and residual austenite cannot beobtained. Therefore, the bainitic transformation treatment temperatureis 500° C. or lower. In order to promote carbon concentration toaustenite with bainitic transformation, the bainitic transformationtreatment temperature is preferably 485° C. or lower and more preferably470° C. or lower. On the other hand, when the bainitic transformationtreatment temperature is lower than 250° C., bainitic transformationdoes not sufficiently proceed and residual austenite cannot be obtained.Therefore, the bainitic transformation treatment temperature is 250° C.or higher. In order to effectively proceed bainitic transformation, thebainitic transformation treatment temperature is preferably 300° C. orhigher and more preferably 340° C. or higher.

After the cooling step before plating, in the case in which both thebainitic transformation treatment and the martensitic transformationtreatment are performed, regarding the treatment order, the martensitictransformation treatment is performed before the bainitic transformationtreatment.

[Plating Step]

Next, the base steel sheet obtained as described above is immersed in aplating bath.

The plating bath mainly includes zinc and has a composition in which theamount of effective Al, which is a value obtained by subtracting thetotal amount of Fe from the total amount of Al in the plating bath, is0.050 to 0.180% by mass. When the amount of effective al in the platingbath is less than 0.050%, the entering of Fe into the plated layerexcessively proceeds to impair plating adhesion. Thus, it is requiredthat the amount of effective Al is 0.050% or more. From this viewpoint,the amount of effective Al in the plating bath is preferably 0.065% ormore and more preferably 0.070% or more. On the other hand, when theamount of effective Al in the plating bath is more than 0.180%, Al-basedoxides are formed at the boundary between the base steel sheet and theplated layer and the movement of Fe and Zn atoms is inhibited at thesame boundary. Thus, ζ phase formation is suppressed and platingadhesion is significantly deteriorated. From this viewpoint, it isrequired that the amount of effective Al in the plating bath is 0.180%or less and the amount of effective Al is preferably 0.150% or less andmore preferably 0.135% or less.

One or two or more elements of Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu,Ge, Hf, I, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, Pb, Rb, Sb, Si, Sn, Sr,Ta, Ti, V, W, Zr, and REM may be mixed in the plating bath and there isa preferable case in which the corrosion resistance or workability ofthe hot-dip galvanized layer is improved according to the content ofeach element or the like.

In addition, the temperature of the plating bath is 450° C. to 470° C.When the temperature of the plating bath is lower than 450° C., theviscosity of the plating bath is excessively increased and thus it isdifficult to control the thickness of the plated layer so that theexternal appearance of the hot-dip galvanized steel sheet is impaired.Accordingly, the temperature of the plating bath is 450° C. or higherand preferably 455° C. or higher. On the other hand, when thetemperature of the plating bath is higher than 470° C., a large amountof fumes is generated, and it is difficult to realize safe production,so that the temperature of the plating bath is 470° C. or lower andpreferably 465° C. or lower.

In addition, the steel sheet temperature when the base steel sheetenters the plating bath is lower than 440° C., it is required to give alarge quantity of heat to the plating bath to stabilize the temperatureof the plating bath at 450° C. or higher, which is practicallyinappropriate. On the other hand, when the steel sheet temperature whenthe base steel sheet enters in the plating bath is higher than 480° C.,it is required to introduce a facility of removing a large quantity ofheat from the plating bath to stabilize the temperature of the platingbath at 470° C. or lower, which is inappropriate in terms of productioncosts. Accordingly, in order to stabilize the temperature of the platingbath, the temperature of the base steel sheet when the base steel sheetenters the plating bath is preferably 440° C. or higher and 480° C. orlower. In addition, in order to control a λphase formation behavior tobe appropriate, it is more preferable that the temperature when the basesteel sheet enters the plating bath is controlled to 450° C. or higherand 470° C. or lower.

The temperature of the plating bath is preferably stabilized at atemperature within a range of 450 to 470° C. When the temperature of theplating bath is not stabilized, the λphase in the plating bath becomesnot uniform, which causes nonuniformity in the external appearance andadhesion of the plated layer. In order to stabilize the temperature ofthe plating bath, it is preferable that the steel sheet temperature whenthe steel sheet enters the plating bath is substantially coincident withthe temperature of the plating bath. Specifically, due to the limit oftemperature controllability of an actual production facility, the steelsheet temperature when the steel sheet enters the plating bath ispreferably controlled to be within a range of the temperature of theplating bath ±4° C. and more preferably controlled to be within a rangeof the temperature of the plating bath ±2° C.

In order to have an appropriate plated amount after immersing the steelsheet in the plating bath, an excessive amount of zinc on the surface ispreferably removed by blowing a high pressure gas mainly includingnitrogen onto the steel sheet surface.

[Cooling Step After Plating]

After the plating bath step, in the cooling step of cooling to roomtemperature after plating, a cooling treatment is controlled such that acooling process to 350° C. satisfies the following Expression (1). Inthis manner, an appropriate amount of ζ phase is obtained in the platedlayer.

In Expression (1), T(t) [° C.] represents a steel sheet temperature,t[second] represents the time elapsed from the time point when the steelsheet is taken out from the plating bath as a starting point, t1[second] represents the time elapsed from the time point when the steelsheet is taken out from the plating bath as a starting point and untilthe steel sheet temperature reaches 350° C., and W*Al [% by mass]represents the amount of effective Al in the plating bath. In addition,ε, θ, and μ each represents constant terms, each of which is 2.62×10⁷,9.13×10³, and 1.0×10⁻¹.

$\begin{matrix}{0.40 \leqq \lbrack {\int_{t\; 0}^{t\; 1}{{ɛ \cdot \exp}{\{ {- \frac{\theta \cdot ( \frac{W_{A\; 1}^{*}}{\mu} )^{0.2}}{T(t)}} \} \cdot {tdt}}}} \rbrack^{0.5} \leqq 2.20} & {{Expression}\mspace{14mu} (1)}\end{matrix}$

The above Expression (1) is an expression related to a ζ phase formationbehavior and as the value of the above Expression (1) increases, ζ phaseformation proceeds in the plated layer. As the steel sheet temperatureincreases and the treatment time increases, the value of the aboveExpression (1) increases. In addition, when the amount of effective Alin the plated layer is increased, the value of the above Expression (1)is decreased and ζ phase formation is inhibited. When the steel sheettemperature is within a temperature range of 350° C. or lower, thediffusion of Fe atoms from the base steel sheet to the plated layerhardly occur and ζ phase formation is nearly stopped. Therefore, theabove Expression (1) is used for calculation at a steel sheettemperature within a range of 350° C. or higher.

In the cooling step after plating which is performed after the immersingthe steel sheet in the plating bath, when the value of the aboveExpression (1) is less than 0.40, a sufficient amount of the ζ phase isnot obtained in the plated layer and plating adhesion is impaired. Whenthe value of the above Expression (1) is 0.40 or more, ζ phase formationis sufficiently promoted and the ratio of the interface between the ζphase and the base steel sheet in the entire interface between thehot-dip galvanized layer and the base steel sheet is 20% or more. Inaddition, when the value of the above Expression (1) is 0.40 or more,the ratio of the interface formed between the ζ grains in which coarseoxides are present and the base steel sheet in the interface between theζ phase and the base steel sheet is 50% or less. Accordingly, in orderto obtain sufficient plating adhesion, it is required that the coolingtreatment is controlled such that the value of the above Expression (1)is 0.40 or more. In order to further enhance plating adhesion, it ispreferable that the cooling treatment is controlled such that the valueof the above Expression (1) is 0.50 or more, and it is more preferablethat the cooling treatment is controlled such that the value of theabove Expression (1) is 0.60 or more. On the other hand, when the valueof the above Expression (1) in the cooling treatment is excessivelylarge, alloying of the plated layer proceeds and the Fe content in theplated layer is excessively increased. Thus, plating adhesion isimpaired. From the viewpoint, it is required that he cooling treatmentis performed such that the value of the above Expression (1) is 2.20 orless. In order to further enhance plating adhesion, it is preferablethat the cooling treatment is controlled such that the value of theabove Expression (1) is 2.00 or less and it is more preferable that thecooling treatment is controlled such that the value of the aboveExpression (1) is 1.80 or less.

Here, when the temperature of the steel sheet is increased after thesteel sheet is taken out from the plating bath, the value of the aboveExpression (1) is significantly increased and plating adhesion isdeteriorated. In addition, the microstructure of the steel sheet isreformed and predetermined residual austenite cannot be obtained andstrength deteriorates. Further, coarse carbides are formed and there isa concern of deterioration in formability. Therefore, the steel sheettemperature after the steel sheet is taken out from the plating bath isnot allowed to be higher than the higher temperature of the steel sheettemperature before the steel sheet is immersed in the plating bath andthe plating bath temperature.

On the other, as shown in a general method of producing a hot-dipgalvanized steel sheet, when the steel sheet is rapidly cooled after thesteel sheet is immersed in the plating bath, the value of the aboveExpression (1) is significantly decreased. As a result, a sufficientamount of the ζ phase is not obtained and plating adhesion isdeteriorated. In order to set the value of the above Expression (1) tobe within a predetermined range, for example, after the steel sheet istaken out from the plating bath, the steel sheet may be subjected to anisothermal retention treatment for a predetermined period of time andthen rapidly cooled.

In addition, as long as the value of the above Expression (1) is set tobe within a predetermined range, another optional temperature controlmay be performed. That is, as long as the temperature control forsetting the value of the above Expression (1) to be within the range ofthe present invention, any cooling control form may be adopted. Forexample, a cooling form of rapidly cooling after an isothermal retentiontreatment may be used or a cooling form of almost constant slow coolingmay be used.

In addition, a bainitic transformation treatment in which the steelsheet is retained within a temperature range of 250° C. to 350° C. toobtain residual austenite may be performed after a sufficient amount ofthe ζ phase is obtained in the plated layer by the cooling treatmentsatisfying the above Expression (1). At a bainitic transformationtreatment temperature of lower than 250° C., bainitic transformationdoes not sufficiently proceed and a sufficient amount of residualaustenite cannot be obtained. Therefore, the bainitic transformationtreatment temperature is 250° C. or higher. The bainitic transformationtreatment temperature is more preferably 300° C. or higher for effectiveprogress of bainitic transformation. On the other hand, when thebainitic transformation treatment temperature is higher than 350° C.,the diffusion of Fe atoms from the base steel sheet to the plated layerexcessively occurs and thus plating adhesion is deteriorated. Therefore,the bainitic transformation treatment temperature is 350° C. or lowerand preferably 340° C. or lower.

In order to further stabilize the residual austenite, the steel sheetmay be cooled to 250° C. or lower and then a reheating treatment may beperformed. The treatment temperature and the treatment time of thereheating treatment may be appropriately selected according to desiredproperties. However, a sufficient effect cannot be obtained at areheating treatment temperature of lower than 250° C. Therefore, thereheating treatment temperature is preferably 250° C. or higher and morepreferably 280° C. or higher. When the reheating treatment temperatureis higher than 350° C., the diffusion of Fe atoms from the base steelsheet to the plated layer excessively occurs and thus plating adhesionis deteriorated. Therefore, the reheating treatment temperature ispreferably 350° C. or lower and more preferably 340° C. or lower.

In addition, when the treatment time of the reheating treatment islonger than 1,000 seconds, the effect of the treatment is saturated andthus the treatment time is preferably 1,000 seconds or shorter. Further,the formation of pearlite and/or coarse cementite is suppressed and thusthe reheating treatment time is more preferably 500 seconds or shorter.

The hot-dip galvanized steel sheet according to the embodiment can beproduced by the above-described production method. However, the presentinvention is not limited to the above embodiment.

For example, in the production method of the above-described embodiment,the case in which the “cooling step after plating” in which the coolingprocess to 350° C. satisfies the above Expression (1) is performed afterthe plating step is exemplified. However, in the cooling step afterplating, the cooling process to 350° C. may not satisfy the aboveExpression (1). In this case, in order to produce the same hot-dipgalvanized steel sheet produced in the case in which the cooling processto 350° C. satisfies the above Expression (1), it is required to controlthe plating bath immersion time to be appropriate.

This is because ζ phase formation is promoted in the plating bath and areaction between Si and/or Mn oxides present on the surface of the basesteel sheet on which the plated layer is formed and Al in the platingbath is promoted by increasing the plating bath immersion time. Throughthe reaction between the oxides and Al in the plating bath, part ofoxides present on the surface of the base steel sheet are reduced andthe number and amount of the Si and/or Mn oxides present on the surfaceof the base steel sheet are reduced.

For obtaining the above effect by increasing the plating bath immersiontime, it is required to set the plating bath immersion time to 3 secondsor longer, preferably 5 seconds or longer, more preferably 7 seconds orlonger, and even more preferably 10 seconds or longer. When the platingbath immersion time is 10 seconds or longer, the same hot-dip galvanizedsteel sheet in the case in which the “cooling step after plating” inwhich the cooling process to 350° C. satisfies the above Expression (1)is performed is obtained. As a result, a plated layer in which the ratioof the interface between the ζ phase and the base steel sheet in theentire interface between the hot-dip galvanized layer and the base steelsheet is 20% or more and the ratio of the interface formed between the ζgrains in which coarse oxides are present and the base steel sheet inthe interface between the ζ phase and the base steel sheet is 50% orless is obtained.

When the hot-dip galvanized steel sheet is produced, in the case ofusing the method in which the plating bath immersion time is 10 secondsor longer, the cooling process to 350° C. may not satisfy the aboveExpression (1) in the cooling step after plating. Accordingly, even in ahot-dip galvanized steel sheet manufacturing line in which theabove-described “cooling step after plating” is not easily performed,the hot-dip galvanized steel sheet according to the embodiment can beeasily produced.

The plating bath immersion time can be determined to be appropriateaccording to the Al content in the plating bath. The plating bathimmersion time is preferably 20 seconds or shorter and more preferably15 seconds or shorter to secure satisfactory productivity.

For example, in the embodiment of the present invention, a coating filmformed of a composite oxide including a phosphorus oxide and/orphosphorus may be applied to the surface of the zinc-plated layer of thehot-dip galvanized steel sheet obtained by the above-described.

The coating film formed of a composite oxide including a phosphorusoxide and/or phosphorus can function as a lubricant when the hot-dipgalvanized steel sheet is worked and can protect the zinc-plated layerformed on the surface of the base steel sheet.

Further, in the embodiment of the present invention, cold rolling may beperformed on the hot-dip galvanized steel sheet cooled at roomtemperature at a rolling reduction of 3.00% or less for shapecorrection.

The method of producing the hot-dip galvanized steel sheet according tothe above-described embodiment of the present invention is preferablyapplied to the production of a hot-dip galvanized steel sheet in whichthe thickness of the base steel sheet is 0.6 mm or more and less than5.0 mm. When the thickness of the base steel sheet is less than 0.6 mm,it is difficult to keep the shape of the base steel sheet flat and thethickness is not appropriate in some cases. In addition, when thethickness of the base steel sheet is 5.0 mm or more, the control ofcooling in the annealing step and the plating step may be difficult.

EXAMPLES

Examples of the present invention will be described. The conditions inthe examples are just an illustration which is employed for confirmingthe feasibility and effects of the present invention. The presentinvention is not limited to this illustration of conditions. The presentinvention can employ various conditions so long as not deviating fromthe gist of the present invention and achieving the object of thepresent invention.

Example 1

Slabs having the chemical components (composition) shown in Tables 1 to4 were casted, hot-rolled under the conditions (the slab heatingtemperature, the rolling completion temperature) for the hot rollingshown in Tables 5 and 6, cooled under the conditions (the averagecooling rate from hot rolling completion to coiling, and the coilingtemperature) for the hot rolling step shown in Tables 5 and 6, and thushot-rolled steel sheets were obtained.

Thereafter, the hot-rolled steel sheets were subjected to pickling andcold rolling under the condition (rolling reduction) for the coldrolling shown in Tables 5 and 6 and thus cold-rolled steel sheets wereobtained.

Next, the obtained cold-rolled steel sheets were subjected to annealingunder the conditions (the air ratio in the preheating zone, thepreheating completion temperature in the preheating zone, the partialpressure ratio (P(H₂O)/P(H₂) between H₂O and H₂ in the reductionatmosphere, the average heating rate in a temperature range of 600° C.to 750° C., the maximum heating temperature Tm) for the heating step inthe annealing step shown in Tables 7 and 8. The preheating completiontemperature of Experimental Examples 1 to 94 was within a range of 623°C. to 722° C.

Subsequently, the steel sheets were subjected to a cooling treatmentunder the conditions (cooling rate 1 (the average cooling rate in atemperature range of 750° C. to 700° C.), cooling rate 2 (the averagecooling rate in a temperature range of 700° C. to 500° C.), theconditions for bainitic transformation treatment 1 (the treatmenttemperature, the treatment time), martensitic transformation treatment(the treatment temperature, the treatment time)) for the cooling stepbefore plating shown in Tables 7 and 8. Regarding the steel sheet whichhad not been subjected to the bainitic transformation treatment 1 andthe martensitic transformation treatment, the columns of the aboveconditions for the treatments were blank.

Next, the steel sheets were immersed in a zinc plating bath under theconditions (the amount of effective Al, the plating bath temperature(bath temperature), the steel sheet entering temperature, the immersiontime) for the plating step shown in Tables 9 and 10 to be plated.

After the plating step, a cooling treatment was performed under theconditions (Expression (1), the conditions (the treatment temperature,the treatment time) for bainitic transformation treatment 2, theconditions (the treatment temperature, the treatment time) for thereheating treatment) for the cooling step after plating shown in Tables9 and 10. Regarding the steel sheet which had not been subjected to thebainitic transformation treatment 2 and the reheating treatment, thecolumns of the conditions for the treatments were blank.

Further, cold rolling was performed under the conditions (rollingreduction) shown in Tables 9 and 10 to obtain plated steel sheets ofExperimental Examples 1 to 94 and C1 to C44 (wherein the experiment wasstopped in some of experimental examples).

In the obtained plated steel sheets (Experimental Examples 1 to 94 andC1 to C44), the microstructure of the base steel sheet and the platedlayer were observed. The observation results of the microstructure ofthe base steel sheet and the plated layer are shown in Tables 11 and 12.

First, a thickness cross section parallel to the rolling direction ofthe base steel sheet was set as an observed section and a sample wascollected from the plated steel sheet. The structure observation with afield emission scanning electron microscope (FE-SEM) and high resolutioncrystal orientation analysis according to an EBSD method were performedon the observed section of the sample. The microstructure in a range of⅛ thickness to ⅜ thickness centered at the position of ¼ of thethickness from the surface of the base steel sheet was observed toidentify the constructional structure. In Tables 11 and 12, F representsgranular ferrite, WF represents needle-like ferrite, NRF representsunrecrystallized ferrite, P represents pearlite, θ represents coarsecementite, BF represents bainitic ferrite, B represents bainite, Mrepresents martensite, tM represents tempered martensite, and γrepresents residual austenite, respectively in the observation.

In addition, a small piece having a size of 25 mm×25 mm was collectedfrom the plated steel sheet as a test piece. In the range of ⅛ thicknessto ⅜ thickness from the surface of the thickness of the test piece, thesurface parallel to the sheet surface was mirror finished and the volumefraction (y fraction) of the residual austenite was measured by an X-raydiffraction method.

Further, a thickness cross section parallel to the rolling direction ofthe base steel sheet was set as an observed section and a sample wascollected from the plated steel sheet. The observed section of thesample was observed with a field emission scanning electron microscope(FE-SEM) to observe the interface between the plated layer and the basesteel sheet. From the result thereof, the ratio of the interface betweenthe ζ phase and the base steel sheet in the interface between the platedlayer and the base steel sheet (boundary surface occupancy ratio), theratio of the interface between the δ1 phase and the base steel sheet inthe interface between the plated layer and the base steel sheet(boundary surface occupancy ratio), and the ratio of the interfaceformed between the ζ grains in which coarse oxides were present and thebase steel sheet in the entire interface between the ζ phase and thebase steel sheet (ratio of boundary surface where oxide presents) wereobtained by the above-described method.

The plated amount of the plating was obtained by melting the platedlayer using a hydrochloric acid with an inhibitor and comparing theweight before and after the melting.

Further, a thickness cross section parallel to the rolling direction ofthe base steel sheet was set as an observed section and a sample wascollected from the plated steel sheet. The average thickness of therefined layer to be in direct contact with the interface between thebase steel sheet and the hot-dip galvanized layer (the average thicknessof the refined layer), the average grain size of the ferrite in therefined layer (the average grain size of the ferrite), and the maximumsize of one or two or more of oxides of Si and Mn in the refined layer(the maximum size of the oxide) were obtained by using theabove-described measurement method. The results are shown in Tables 11and 12.

Next, in order to investigate the properties of each plated steel sheet,a tensile test, a hole expansion test, a bending test, an adhesionevaluation test, a spot welding test, a corrosion test, a chippingproperty test, and a powdering property test were performed. Theproperties in each experimental example are shown in Tables 13 and 14.

No. 5 test pieces as described in JIS Z 2201 were cut out from theplated steel sheets to perform a tensile test according to the methoddescribed in JIS Z2241. Thus, the yield strength YS, the maximum tensilestrength TS, and the total elongation El were obtained. The tensileproperties were evaluated such that case in which the maximum tensilestrength TS was 420 MPa or more was satisfactory.

A hole expansion test was performed according to the method described inJIS Z 2256. The ductility (total elongation) El and hole expansibility λof formability changes according to the maximum tensile strength TS.However, the strength, the ductility and the hole expansibility in thecase in which the following Expression (3) was satisfied weresatisfactory.

TS ^(1.5) ×El×λ ^(0.5)≧2.5×10⁶   Expression (3)

For plating adhesion, each plated steel sheet to which 5% uniaxialtension strain was applied was subjected to a DuPont impact test. Anadhesive tape was attached to the plated steel sheet after the impacttest and then peeled off. The case in which the plating was not peeledoff was evaluated as pass (o) and the case in which the plating waspeeled off was evaluated as fail (x). The DuPont impact test wasperformed by dropping a weight of 3 kg onto the steel sheet from aheight of 1 m using a punching die having a radius of curvature of thefront end of ½ inches.

Spot weldability was evaluated by performing a continuous dotting test.Under the condition that the diameter of the welded part is 5.3 to 5.7times the square root of the thickness, spot welding was continuouslyperformed 1, 000 times and d₁ of the first dot and d₁₀₀₀ of the 1,000-thdot of the diameters of the welded parts were compared to each other.The case in which d₁₀₀₀/d₁ was 0.90 or more was evaluated as pass (o)and the case in which d₁₀₀₀/d₁ was less than 0.90 was evaluated as fail(x).

For the evaluation of corrosion resistance, a test piece cut out fromeach plated steel sheet to have a size of 150×70 mm was used. The testpiece was subjected to a zinc phosphate-based dipping type chemicalconversion treatment and subsequently a cation electrodeposition coat of20 μm was applied. Further, an intermediate coat of 35 μm and an uppercoat of 35 μm were applied and then the rear surface and the end portionwere sealed with an insulating tape. In the corrosion resistance test,CCT having one cycle of SST 6 hr→drying 4 hr→wetting 4 hr→freezing 4 hrwas used. The evaluation of corrosion resistance after coating wasperformed such that the coated surface was cross-cut with a cutter untilthe cutting reached the base steel sheet and a swollen width after 60cycles of CCT was measured. The case in which the swollen width was 3.0mm or less was evaluated as pass (o) and the case in which the swollenwidth was more than 3.0 mm was evaluated as fail (x).

Chipping properties were evaluated using a test piece cut out from eachplated steel sheet to have a size of 70 mm×150 mm. First, each step offormation of an automotive degreasing and chemical conversion coatingfilm and 3-coat coating was performed on the test piece. Next, in astate in which the test piece was cooled and retained at −20° C., tencrushed stones (0.3 to 0.5 g) were vertically applied with an airpressure of 2 kgf/cm². The application of the crushed stones wasrepeated five times to each test piece. Then, in each test piece, 50chipping scars in total were observed and evaluation was made based onthe following criteria according to the position of the peeledinterface. The case in which the peeled interface was above the platedlayer (the interface between the plated layer and the chemicalconversion coating film or the interface between the electrodepositioncoat and the intermediate coat coating) was evaluated as (o) and thecase in which even one interface peeling occurred at interface betweenthe plated layer and the base metal was evaluated as (x).

Powdering properties were evaluated using V bending (JIS Z 2248) toevaluate the workability of the plated layer. Each plated steel sheetwas cut into a size of 50×90 mm and a formed body was formed with a1R-90° V-shaped die press to be used as a test piece. In the grooves ofeach test piece, tape peeling was performed. Specifically, a cellophanetape having a width of 24 mm was pressed on the bent part of the testpiece and then peeled off. The part of the cellophane tape at a lengthof 90 mm was visually determined. The evaluation criteria were asfollows. The case in which the peeling of the plated layer occurred inan area of less than 5% of the worked part area was evaluated as (o) andthe case in which the peeling of the plated layer occurred in an area ofmore than 5% of the worked part area was evaluated as (x).

TABLE 1 Chemical components (% by mass) Remainder: Fe and unavoidableimpurities C Si Mn P S Al N O Ti Nb V Remarks A 0.096 0.56 2.57 0.0100.0020 0.031 0.0038 0.0011 Example B 0.081 0.61 1.91 0.013 0.0024 0.1210.0042 0.0025 Example C 0.231 1.50 2.43 0.008 0.0024 0.058 0.0081 0.0019Example D 0.249 0.41 2.02 0.004 0.0014 0.016 0.0030 0.0024 Example E0.144 0.62 1.80 0.008 0.0007 0.069 0.0071 0.0014 0.068 Example F 0.1650.78 1.84 0.016 0.0042 0.022 0.0046 0.0014 0.018 Example G 0.196 0.062.05 0.008 0.0016 1.034 0.0031 0.0009 0.179 Example I 0.175 0.78 1.700.009 0.0022 0.057 0.0024 0.0006 Example J 0.133 0.60 3.35 0.020 0.00110.023 0.0044 0.0023 Example K 0.176 0.44 2.35 0.004 0.0008 0.072 0.00460.0019 Example L 0.138 0.64 1.66 0.019 0.0007 0.016 0.0042 0.0010Example N 0.339 0.50 1.78 0.014 0.0035 0.037 0.0026 0.0007 Example O0.154 0.75 3.35 0.019 0.0023 0.077 0.0008 0.0026 Example P 0.093 0.681.60 0.017 0.0016 0.070 0.0016 0.0029 Example Q 0.151 0.89 2.11 0.0080.0037 0.052 0.0025 0.0024 Example R 0.197 0.49 1.49 0.010 0.0002 0.0560.0047 0.0025 Example S 0.100 1.84 1.90 0.004 0.0006 0.046 0.0039 0.0024Example T 0.209 0.14 3.05 0.013 0.0010 1.168 0.0025 0.0005 0.009 0.028Example U 0.182 0.73 2.63 0.032 0.0033 0.029 0.0020 0.0027 Example V0.109 1.06 2.90 0.013 0.0003 0.072 0.0024 0.0022 0.017 Example W 0.1380.37 0.85 0.011 0.0036 0.310 0.0020 0.0027 0.009 0.041 Example X 0.1301.00 1.41 0.014 0.0038 0.027 0.0017 0.0024 0.017 Example Y 0.147 0.602.53 0.004 0.0069 0.046 0.0017 0.0006 0.026 Example Z 0.187 0.79 1.720.008 0.0015 0.055 0.0040 0.0031 0.009 0.006 0.033 Example AD 0.090 2.222.09 0.010 0.0033 0.019 0.0019 0.0011 0.019 Example AI 0.062 0.80 2.220.018 0.0023 0.062 0.0032 0.0026 0.065 Example AK 0.184 0.48 2.25 0.0030.0037 0.056 0.0032 0.0029 0.046 0.013 Example AS 0.158 1.40 1.84 0.0180.0046 0.058 0.0046 0.0005 Example AT 0.108 0.55 1.21 0.014 0.0031 0,7200.0031 0.0028 0.044 Example AW 0.107 0.95 0.64 0.012 0.0009 0.017 0.00380.0016 0.052 Example BC 0.174 3.25 1.90 0.020 0.0034 0.036 0.0037 0.0033Comp.Ex. BE 0.126 0.68 3.77 0.019 0.0032 0.008 0.0052 0.0013 Comp.Ex. BF0.100 0.53 1.96 0.137 0.0009 0.032 0.0037 0.0015 Comp.Ex. BH 0.145 0.971.95 0.010 0.0012 2.071 0.0007 0.0025 Comp.Ex.

TABLE 2 Chemical components (% by mass) Remainder: Fe and unavoidableimpurities Cr Ni Cu Mo B W Ca Ce Mg Zr La REM Si + 0.7 Al Remarks A 0.58Example B 0.69 Example C 1.54 Example D 0.42 Example E 0.67 Example F0.80 Example G 0.78 Example I 0.72 0.82 Example J 0.67 0.61 Example K0.20 0.49 Example L 0.0010 0.65 Example N 0.0031 0.52 Example O 0.00080.80 Example P 0.0034 0.73 Example Q 0.0021 0.92 Example R 0.0012 0.53Example S 0.0018 1.87 Example T 0.96 Example U 0.79 0.56 0.75 Example V0.0025 1.11 Example W 0.63 0.03 0.0037 0.59 Example X 0.30 1.02 ExampleY 0.0017 0.0018 0.63 Example Z 0.14 0.0004 0.0004 0.83 Example AD 2.23Example AI 0.40 0.84 Example AK 0.31 0.52 Example AS 0.0036 1.44 ExampleAT 1.06 Example AW 0.80 0.09 0.0006 0.96 Example BC 3.28 Comp.Ex. BE0.68 Comp.Ex. BF 0.55 Comp.Ex. BH 2.42 Comp.Ex.

TABLE 3 Chemical components (% by mass) Remainder: Fe and unavoidableimpurities C Si Mn P S Al N O Ti Nb V Remarks CA 0.091 0.50 2.47 0.0040.0041 0.091 0.0040 0.0009 Example CB 0.085 0.69 1.41 0.010 0.0022 0.1030.0032 0.0005 Example CC 0.231 1.50 2.43 0.008 0.0024 0.058 0.00810.0019 Example CD 0.109 1.21 1.65 0.010 0.0024 0.031 0.0021 0.0005Example CE 0.100 0.17 2.43 0.013 0.0019 0.072 0.0038 0.0013 Comp.Ex. CF0.141 0.33 1.39 0.011 0.0043 0.003 0.0004 0.0002 Example CG 0.408 0.832.06 0.008 0.0032 0.031 0.0038 0.0006 Comp.Ex. CH 0.031 0.65 2.36 0.0130.0020 0.045 0.0027 0.0002 Comp.Ex. CI 0.150 0.65 0.33 0.013 0.00230.051 0.0034 0.0015 Comp.Ex. CJ 0.099 0.75 1.90 0.012 0.0136 0.0310.0037 0.0017 Comp.Ex. CK 0.102 0.65 2.45 0.011 0.0013 0.143 0.01980.0010 Comp.Ex. CL 0.155 0.75 1.73 0.009 0.0033 0.043 0.0038 0.0133Comp.Ex.

TABLE 4 Chemical components (% by mass) Remainder: Fe and unavoidableimpurities Cr Ni Cu Mo B W Ca Ce Mg Zr La REM Si + 0.7 Al Remarks CA0.59 Example CB 0.38 0.79 Example CC 1.56 Example CD 1.10 1.24 ExampleCE 0.24 Comp.Ex. CF 0.33 Example CG 0.86 Comp.Ex. CH 0.70 Comp.Ex. CI0.70 Comp.Ex. CJ 0.78 Comp.Ex. CK 0.79 Comp.Ex. CL 0.79 Comp.Ex.

TABLE 5 Hot rolling step Slab heating Rolling completion Cold rollingstep Experimental Chemical temperature temperature Average cooling rateCoiling temperature Rolling reduction Example components ° C. ° C. °C./sec ° C. % Remarks 1 A 1220 954 23 597 50 Example 2 A 1230 913 17 55950 Example 3 A 1235 906 14 601 50 Comp.Ex. 4 B 1190 934 29 583 64Example 5 B 1220 911 15 604 29 Example 6 B 1220 928 16 607 — Comp.Ex. 7C 1190 888 27 584 42 Example 9 C 1195 873 13 600 42 Comp.Ex. 10 D 1240914 44 600 52 Example 12 D 1215 944 35 587 34 Example 13 E 1210 918 12660 43 Example 14 F 1240 868 28 558 32 Example 15 G 1205 900 57 560 50Example 17 I 1200 949 24 562 44 Example 18 J 1210 921 63 587 42 Example20 J 1200 927 16 583 55 Example 21 K 1235 911 23 554 46 Example 22 L1210 927 19 610 50 Example 25 N 1210 904 63 553 0.14 Example 26 N 1245941 61 572 55 Comp.Ex. 27 O 1235 896 17 542 39 Example 28 P 1185 961 13668 44 Example 29 Q 1180 938 36 563 46 Example 30 R 1185 915 31 574 64Example 31 S 1215 888 28 496 68 Example 33 S 1190 898 30 565 55 Comp.Ex.34 T 1195 899 18 614 65 Example 35 T 1210 894 56 566 89 Comp.Ex. 36 U1190 907 57 385 59 Example 37 V 1205 915 12 657 53 Example 38 W 1225 90546 566 41 Example 39 X 1235 872 24 589 53 Example 40 X 1230 940 27 54357 Comp.Ex. 41 Y 1195 897 16 544 37 Example 42 Z 1185 924 22 579 42Example 43 Z 1245 952 28 591 57 Comp.Ex. 55 AD 1235 893 17 542 58Example 62 AI 1215 894 22 555 49 Example 65 AK 1235 899 20 597 3.4Example 76 AS 1255 880 48 387 50 Example 78 AT 1190 947 17 670 42Example 82 AW 1210 929 40 710 50 Example 89 BC Experiment stopped due tooccurrence of cracking of slab during cooling Comp.Ex. 91 BE Experimentstopped due to occurrence of cracking of slab during heating Comp.Ex. 92BF Experiment stopped due to occurrence of cracking of slab duringheating Comp.Ex. 94 BH Experiment stopped due to occurrence of crackingof slab during cooling Comp.Ex.

TABLE 6 Hot rolling step Slab heating Rolling completion Cold rollingstep Experimental Chemical temperature temperature Average cooling rateCoiling temperature Rolling reduction Example components ° C. ° C. °C./sec ° C. % Remarks C1 CA 1225 948 23 589 50 Example C2 CA 1220 938 25609 65 Example C3 CA 1220 912 21 564 81 Example C4 CA 1205 950 20 604 50Comp.Ex. C5 CA 1220 918 20 609 50 Comp.Ex. C6 CA 1205 910 20 559 50Example C7 CA 1215 940 20 560 40 Example C8 CA 1225 928 21 598 50Comp.Ex. C9 CA 1200 915 25 571 50 Comp.Ex. C10 CA 1230 903 16 592 50Comp.Ex. C11 CB 1185 933 33 574 74 Example C12 CB 1215 909 18 595 29Example C13 CB 1210 910 20 593 50 Example C14 CB 1180 933 23 547 50Example C15 CB 1215 928 31 591 60 Comp.Ex. C16 CB 1200 920 19 584 60Comp.Ex. C17 CB 1210 901 19 559 50 Comp.Ex. C18 CB 1230 821 17 605 —Comp.Ex. C19 CC 1200 891 27 576 42 Example C20 CC 1230 927 66 575 42Example C21 CC 1210 893 29 576 50 Example C22 CC 1220 904 21 249 —Comp.Ex. C23 CC 1195 878 15 604 42 Comp.Ex. C24 CC 1205 900 25 590 50Comp.Ex. C25 CC 1220 917 34 585 50 Comp.Ex. C26 CC 1210 915 30 605 50Comp.Ex. C27 CC 1195 909 24 558 50 Example C28 CD 1220 950 14 648 57Comp.Ex. C29 CD 1210 936 36 637 50 Example C30 CD 1215 937 37 629 50Comp.Ex. C31 CD 1200 906 19 604 50 Example C32 CD 1195 901 23 598 50Example C33 CD 1230 899 20 605 50 Example C34 CE 1200 885 30 550 50Comp.Ex. C35 CF 1240 885 28 565 59 Example C36 CF 1220 896 24 567 59Example C37 CF 1210 900 20 588 59 Comp.Ex. C38 CG 1215 906 30 598 47Comp.Ex. C39 CH 1225 914 33 593 59 Comp.Ex. C40 CI 1200 902 20 550 50Comp.Ex. C41 CJ 1235 927 31 605 64 Comp.Ex. C42 CK 1250 913 15 558 53Comp.Ex. C43 CL 1185 894 30 568 53 Comp.Ex. C44 CC 1195 901 29 586 42Example

TABLE 7 Annealing step Heating step Maxi- Cooling step before platingmum Bainite Martensite heating Reduc- Treat- Treat- Experi- Chemi-Average temper- Pre- tion ment Treat- ment Treat- mental cal heatingature Tm- heating zone Cooling Cooling temper- ment temper- ment Exam-compo- rate Tm Acl Acl zone P(H₂O)/ rate 1 rate 2 ature time ature timeple nents ° C./sec ° C. ° C. ° C. Air ratio P(H₂) ° C./sec ° C./sec ° C.sec ° C. sec Remarks 1 A 2.9 813 715 98 0.9 0.85 2.0 33.9 Example 2 A1.4 773 715 58 1.0 0.57 1.2 9.4 464 49 Example 3 A 4.1 861 715 146 1.00.76 2.3 23.3 469 33 Comp.Ex. 4 B 2.4 875 721 154 0.9 0.71 1.3 23.0 405115 Example 5 B 1.9 776 721 55 1.0 0.006 1.7 23.3 450 57 Example 6 BExperiment stopped due to disablement of annealing treatment by shapedefect of steel sheet Comp.Ex. 7 C 2.4 807 751 56 1.1 0.63 2.2 13.5 47352 Example 9 C 4.3 815 751 64 0.4 0.54 3.0 11.4 Comp.Ex. 10 D 4.4 875717 158 0.9 0.84 2.4 12.7 451 59 Example 12 D 5.1 831 717 114 1.0 0.783.3 33.3 367 238 156 15 Example 13 E 3.5 838 717 121 1.0 0.60 2.2 30.3460 28 Example 14 F 1.6 844 731 113 0.9 0.47 1.0 9.4 486 43 Example 15 G3.4 868 765 103 0.9 0.47 1.9 15.2 433 39 Example 17 I 6.0 875 714 1611.0 0.59 3.3 12.6 465 125 Example 18 J 2.8 785 716 69 0.9 0.70 2.2 11.8430 58 Example 20 J 1.6 849 716 133 0.9 0.89 1.1 7.4 371 231 Example 21K 2.9 849 712 137 1.1 0.72 2.0 19.3 327 86 Example 22 L 1.4 877 715 1620.9 0.50 1.8 6.2 413 35 Example 25 N 2.4 781 716 65 1.1 0.58 2.0 15.0446 176 Example 26 N 2.0 873 716 157 0.9 0.80 1.1 10.3 423 29 Comp.Ex.27 O 2.0 774 716 58 1.1 0.77 1.9 18.5 Example 28 P 4.6 803 723 80 1.00.77 3.4 24.3 Example 29 Q 3.5 900 737 163 1.0 0.75 1.7 16.9 380 87Example 30 R 3.1 789 733 56 1.1 1.00 2.5 18.7 447 29 Example 31 S 2.8893 766 127 0.9 0.62 1.4 7.6 Example 33 S 0.3 847 766 81 0.9 0.73 1.429.7 461 32 Comp.Ex. 34 T 4.2 834 762 72 0.9 0.79 2.7 26.7 472 27Example 35 T Experiment stopped due to breaking of sheet by cold rollingComp.Ex. 36 U 3.3 844 710 134 1.0 0.57 2.0 10.5 Example 37 V 2.3 823 73192 0.8 0.86 1.5 12.6 444 301 Example 38 W 1.3 814 761 53 0.8 0.73 1.38.9 389 22 Example 39 X 3.6 824 739 85 1.0 0.73 2.4 14.8 476 28 Example40 X 3.8 839 739 100 0.9 2.35 2.0 10.4 461 35 Comp.Ex. 41 Y 2.6 774 71856 1.0 0.51 2.3 8.7 Example 42 Z 2.5 843 724 119 1.0 0.49 1.5 12.1 49160 Example 43 Z 2.3 821 724 97 1.5 0.55 1.5 10.3 448 57 Comp.Ex. 55 AD1.2 830 775 55 1.0 0.48 1.0 12.1 Example 62 AI 2.4 784 722 62 1.0 0.861.3 28.9 445 29 Example 65 AK 2.5 899 707 192 1.0 1.55 1.2 18.6 475 41Example 76 AS 3.2 882 758 124 1.0 0.13 1.7 13.3 405 217 Example 78 AT3.4 827 768 59 1.0 0.76 2.2 30.3 470 21 Example 82 AW 2.3 912 756 1560.9 1.25 1.1 10.8 Example 89 BC Experiment stopped due to occurrence ofcracking of slab during cooling Comp.Ex. 91 BE Experiment stopped due tooccurrence of cracking of slab during heating Comp.Ex. 92 BF Experimentstopped due to occurrence of cracking of slab during heating Comp.Ex. 94BH Experiment stopped due to occurrence of cracking of slab duringcooling Comp.Ex.

TABLE 8 Annealing step Heating step Maxi- Preheating Cooling step beforeplating mum zone Bainite Martensite heating Comple- Reduc- Treat- Treat-Experi- Chemi- Average temper- tion tion ment Treat- ment Treat- mentalcal heating ature Tm- Air temper- zone Cooling Cooling temper- menttemper- ment Exam- compo- rate Tm Acl Acl ratio ature P(H₂O)/ rate 1rate 2 ature time ature time ple nents ° C./sec ° C. ° C. ° C. ° C. ° C.P(H₂) ° C./sec ° C./sec ° C. sec ° C. sec Remarks C1 CA 2.9 803 685 1180.9 669 0.85 12.2 30.9 Example C2 CA 23.0 767 685 82 0.9 641 0.63 1.513.0 285 47 Example C3 CA 2.0 783 685 98 1.0 697 0.59 1.4 10.4 464 48Example C4 CA 2.0 741 685 56 0.9 656 0.88 2.2 28.0 460 52 Comp.Ex. C5 CA2.5 810 685 125 0.9 671 0.81 0.3 10.3 427 72 Comp.Ex. C6 CA 2.3 790 685105 0.9 723 0.0008 1.5 12.0 460 58 Example C7 CA 2.9 803 685 118 0.9 7010.83 7.2 62.9 412 10 Example C8 CA 2.5 810 685 125 0.9 670 0.80 2.2 29.0Comp.Ex. C9 CA 3.1 785 685 100 0.9 657 0.62 1.6 11.3 476 12 Comp.Ex. C10CA 4.1 861 685 176 1.0 664 0.76 2.3 23.3 469 33 Comp.Ex. C11 CB 2.4 835713 122 0.9 598 0.71 1.3 23.0 405 115 Example C12 CB 2.0 781 713 68 0.9700 0.004 2.0 20.0 465 68 Example C13 CB 3.1 790 713 77 0.9 775 0.28 1.820.3 407 19 Example C14 CB 2.6 825 713 112 0.9 716 0.77 1.5 18.6 398 23Example C15 CB 2.4 805 713 92 0.9 685 0.65 1.6 3.5 445 105 Comp.Ex. C16CB 2.5 825 713 112 0.8 685 0.65 1.2 25.0 Comp.Ex. C17 CB 2.8 815 713 1020.9 692 0.73 1.8 18.8 417 105 Comp.Ex. C18 CB Experiment stopped due todisablement of cold rolling by shape defect of hot-rolled steel sheetComp.Ex. C19 CC 2.4 807 738 69 1.0 627 0.63 2.2 13.5 473 52 Example C20CC 2.0 793 738 55 0.9 739 0.64 1.5 43.9 Example C21 CC 2.5 802 738 640.9 636 0.58 2.0 14.9 479 58 Example C22 CC Experiment stopped due tobreaking of steel sheet in cold rolling step Comp.Ex. C23 CC 3.4 818 73880 0.9 711 0.0000 3.0 12.0 Comp.Ex. C24 CC 2.0 809 738 71 0.9 711 0.602.2 13.5 Comp.Ex. C25 CC 2.0 801 738 63 0.9 662 0.60 2.0 36.2 555 74Comp.Ex. C26 CC 2.3 799 738 61 0.9 710 0.59 3.1 25.6 235 52 Comp.Ex. C27CC 1.5 854 738 116 1.0 715 0.60 10.7 49.3 480 30 Example C28 CD 2.5 776744 32 0.8 652 0.65 2.1 12.0 439 42 Comp.Ex. C29 CD 2.5 806 744 62 0.8665 0.68 1.9 13.3 438 59 Example C30 CD 2.5 810 744 66 0.9 630 0.65 2.012.8 470 7 Comp.Ex. C31 CD 2.3 810 744 66 0.8 637 0.59 2.5 20.3 385 14Example C32 CD 2.5 800 744 56 1.0 647 0.63 21.9 22.5 440 18 Example C33CD 2.4 807 744 63 0.9 708 0.67 2.1 13.4 444 60 Example C34 CE 3.0 809709 100 0.9 633 0.58 2.2 16.0 451 64 Comp.Ex. C35 CF 3.0 800 706 94 1.0639 0.60 2.5 16.3 453 64 Example C36 CF 2.7 804 706 98 1.0 673 0.63 2.417.0 458 58 Example C37 CF 2.9 801 706 95 1.0 620 0.62 2.5 14.0 465 743Comp.Ex. C38 CG 2.4 788 689 99 0.9 687 0.82 1.5 14.3 444 84 Comp.Ex. C39CH 3.7 839 704 135 1.0 656 0.78 2.6 18.1 471 73 Comp.Ex. C40 CI 2.3 810738 72 0.8 619 0.65 1.8 24.5 461 71 Comp.Ex. C41 CJ 3.5 828 719 109 0.9648 0.84 2.0 15.4 470 68 Comp.Ex. C42 CK 4.7 805 713 92 0.8 665 0.60 3.517.3 471 75 Comp.Ex. C43 CL 3.3 812 733 79 0.8 723 0.79 2.6 15.9 468 83Comp.Ex. C44 CC 2.5 807 738 69 1.0 813 0.23 2.5 15.0 475 55 Comp.Ex.

TABLE 9 Plating step Plating bath Amount Cooling step after plating ofSteel Bainite Reheating treatment Cold effec- sheet Treat- Treat-rolling tive Bath entering Immer- ment Treat- ment Treat- RollingExperi- Al temper- temper- sion Expres- temper- ment temper- ment reduc-mental % by ature ature time sion ature time ature time tion Examplemass ° C. ° C. sec (1) ° C. sec ° C. sec % Remarks 1 0.104 459 459 3.00.62 338 57 0.16 Example 2 0.144 465 445 4.6 0.42 0.20 Example 3 0.112456 456 3.0 0.33 0.23 Comp.Ex. 4 0.098 463 454 5.2 0.65 0.11 Example 50.099 460 445 5.2 0.57 0.12 Example 6 Experiment stopped due todisablement of annealing treatment by shape defect of steel sheetComp.Ex. 7 0.087 463 464 7.5 10.50 0.00 Example 9 0.096 461 457 7.4 8.80327 151 0.23 Comp.Ex. 10 0.106 452 463 5.2 9.20 0.21 Example 12 0.098462 469 6.7 9.00 0.16 Example 13 0.090 457 460 6.9 9.00 0.09 Example 140.086 458 459 3.6 8.70 0.17 Example 15 0.088 455 462 6.6 11.10 0.20Example 17 0.097 460 469 3.4 9.40 0.15 Example 18 0.090 459 464 9.014.20 0.00 Example 20 0.102 455 454 5.4 4.70 275 25 0.18 Example 210.092 458 456 6.1 8.70 0.12 Example 22 0.105 464 466 3.8 16.20 331 270.08 Example 25 0.097 459 444 3.5 12.50 0.26 Example 26 0.080 464 4733.0 11.80 0.09 Comp.Ex. 27 0.095 461 460 5.7 12.20 314 17 0.18 Example28 0.107 460 460 8.3 11.10 341 27 264 16 0.19 Example 29 0.114 464 4716.5 7.00 0.07 Example 30 0.100 458 464 4.8 5.20 0.12 Example 31 0.100462 457 9.6 16.80 290 51 0.22 Example 33 0.088 459 461 6.4 14.60 0.18Comp.Ex. 34 0.097 459 457 5.2 4.90 0.07 Example 35 Experiment stoppeddue to breaking of sheet by cold rolling Comp.Ex. 36 0.086 460 465 4.81.19 326 264 0.25 Example 37 0.103 460 456 7.8 0.69 0.09 Example 380.090 458 463 3.6 1.24 293 160 0.05 Example 39 0.090 467 456 3.2 0.980.17 Example 40 0.093 456 462 4.1 0.87 0.05 Comp.Ex. 41 0.100 458 4608.3 0.90 334 48 0.18 Example 42 0.093 460 465 7.6 1.03 0.14 Example 430.097 462 469 6.8 1.00 0.26 Comp.Ex. 55 0.110 463 459 7.8 0.77 308 230.18 Example 62 0.103 455 451 5.5 0.75 0.09 Example 65 0.092 459 449 5.90.64 0.10 Example 76 0.086 461 443 5.8 0.67 0.07 Example 78 0.099 466456 9.2 0.72 0.12 Example 82 0.100 457 478 5.3 0.97 308 40 0:06 Example89 Experiment stopped due to occurrence of cracking of slab duringcooling Comp.Ex. 91 Experiment stopped due to occurrence of cracking ofslab during heating Comp.Ex. 92 Experiment stopped due to occurrence ofcracking of slab during heating Comp.Ex. 94 Experiment stopped due tooccurrence of cracking of slab during cooling Comp.Ex.

TABLE 10 Plating step Plating bath Amount of Steel effec- sheet Coolingstep after plating Cold Experi- tive Bath entering Immer- BainiteReheating treatment rolling mental Al temper- temper- sion Expres-Treatment Treatment Treatment Treatment Rolling Exam- % by ature aturetime sion temperature time temperature time reduction ple mass ° C. ° C.sec (1) ° C. sec ° C. sec % Remarks C1 0.104 459 459 9.5 0.62 338 570.16 Example C2 0.102 460 458 6.5 0.61 0.09 Example C3 0.103 460 457 5.00.60 0.12 Example C4 0.103 460 459 4.7 0.65 0.11 Comp.Ex. C5 0.104 462459 3.3 0.68 0.10 Comp.Ex. C6 0.102 462 460 4.8 0.63 0.10 Example C70.131 467 463 3.3 0.52 342 38 0.06 Example C8 0.088 469 469 6.9 2.37 398164 0.10 Comp.Ex. C9 0.102 460 460 6.5 0.62 0.08 Comp.Ex. C10 0.034 464464 8.9 3.87 0.10 Comp.Ex. C11 0.098 463 454 3.7 0.65 0.11 Example C120.096 454 450 9.3 0.59 0.10 Example C13 0.098 455 453 6.5 0.60 0.10Example C14 0.072 469 469 4.2 1.82 0.11 Example C15 0.100 460 459 4.80.60 0.11 Comp.Ex. C16 0.099 458 455 5.1 0.63 235 105 0.11 Comp.Ex. C170.236 460 459 7.9 0.15 0.10 Comp.Ex. C18 Experiment stopped due todisablement of cold rolling by shape defect of hot-rolled steel sheetComp.Ex. C19 0.087 463 464 4.8 0.81 0.00 Example C20 0.102 461 446 7.30.51 328 113 0.00 Example C21 0.090 462 466 5.4 0.77 310 781 0.03Example C22 Experiment stopped due to breaking of steel sheet in coldrolling step Comp.Ex. C23 0.094 461 457 5.7 0.63 347 183 0.10 Comp.Ex.C24 0.092 462 464 3.4 0.71 0.10 Comp.Ex. C25 0.100 458 456 7.1 0.58 0.10Comp.Ex. C26 0.100 460 457 8.6 0.58 0.08 Comp.Ex. C27 0.067 468 467 8.91.73 336 26 0.05 Example C28 0.106 461 459 6.1 0.61 0.11 Comp.Ex. C290.104 456 459 4.0 0.58 0.11 Example C30 0.104 456 459 7.8 0.58 315 60.10 Comp.Ex. C31 0.106 465 464 4.4 0.70 347 9 0.11 Example C32 0.103459 460 7.9 0.61 338 38 317 17 0.09 Example C33 0.162 456 459 8.5 0.430.10 Example C34 0.107 460 459 3.1 0.62 0.09 Comp.Ex. C35 0.091 462 4594.2 0.55 0.14 Example C36 0.054 461 460 6.5 2.08 0.14 Example C37 0.090463 459 9.3 0.58 0.10 Comp.Ex. C38 0.089 462 461 3.5 0.89 0.08 Comp.Ex.C39 0.121 466 476 7.3 0.91 0.10 Comp.Ex. C40 0.109 459 459 5.8 0.98 0.10Comp.Ex. C41 0.108 456 458 4.7 0.64 0.10 Comp.Ex. C42 0.111 456 455 5.90.63 0.10 Comp.Ex. C43 0.120 456 455 4.4 0.78 0.10 Comp.Ex. C44 0.089464 463 5.1 0.83 0.00 Comp.Ex.

TABLE 11 Plated layer ζ Phase Ratio Base steel sheet of δ1Phase Aver-Aver- Bound- bound- Bound- age age ary ary ary thick- grain Maxi-Microstructure surface surface surface Plat- ness size mum Experi- Chem-γ occu- where occu- ed of of size mental ical Frac- Content pancy oxidespancy a- refined ferrite of Exam- compo- Constitutional tion Fe Al ratiopresent ratio mount layer phase oxide ple nents structure % % % % % %g/m² μm μm μm Remarks 1 A F, BF, M, γ 3 1.7 0.34 62 0 0 61 3.1 0.8 0.03Example 2 A F, BF, B, M, γ 5 0.8 0.31 22 0 0 58 2.7 0.4 0.1 Example 3 AF, BF, B, M, γ 2 0.3 0.23  3 0 0 64 2.6 0.5 0.04 Comp.Ex. 4 B F, BF, B,γ 6 1.2 0.31 37 0 0 72 3.3 1.2 0.02 Example 5 B F, BF, B, M, γ 2 1.80.18 60 31 0 58 1.0 1.4 0.4 Example 6 B Experiment stopped due todisablement of annealing treatment by shape defect of steel sheetComp.Ex. 7 C F, BF, γ 8 2.2 0.18 71 0 0 48 2.0 2.4 0.03 Example 9 C F,BF, M, γ 4 0.4 0.20 16 25 0 77 1.0 1.0 0.2 Comp.Ex. 10 D WF, BF, B, M, γ3 1.0 0.27 29 0 0 37 3.4 0.4 0.03 Example 12 D F, BF, B, tM, γ 3 1.60.28 52 0 0 38 3.2 2.3 0.02 Example 13 E F, BF, B, M, γ 5 1.9 0.27 64 00 63 2.8 0.4 0.1 Example 14 F F, BF, B, M, γ 4 2.2 0.19 89 0 0 61 2.30.4 0.04 Example 15 G F, BF, γ 8 3.4 0.23 94 0 0 35 4.0 0.7 0.02 Example17 I F, BF, M, γ 10 1.4 0.20 34 0 0 76 1.9 0.4 0.03 Example 18 J F, BF,B, M, γ 4 1.7 0.22 45 0 0 45 2.4 0.3 0.04 Example 20 J BF, B, tM, M, γ 21.6 0.28 40 0 0 60 3.2 0.3 0.04 Example 21 K F, B, tM, M, γ 3 1.9 0.2856 0 0 61 3.2 0.3 0.1 Example 22 L F, BF, tM, γ 5 2.1 0.20 64 0 0 64 2.50.4 0.03 Example 25 N F, BF, B, γ 11 1.5 0.19 48 0 0 74 3.0 0.8 0.03Example 26 N F, WF, B, M, γ 7 5.9 0.26 55 0 45 33 3.1 0.9 0.04 Comp.Ex.27 O F, M, γ 11 1.6 0.29 60 0 0 50 2.5 0.3 0.03 Example 28 P F, tM, γ 40.7 0.34 25 0 0 76 2.6 1.4 0.02 Example 29 Q F, BF, γ 11 1.5 0.23 44 0 059 3.0 0.3 0.1 Example 30 R F, BF, M, γ 5 1.2 0.33 59 0 0 63 3.0 0.3 0.1Example 31 S F, BF, M, γ 4 1.1 0.21 31 0 0 72 2.0 2.4 0.02 Example 33 SF, BF, M, γ 5 1.6 0.20 48 0 0 50 1.8 1.3 0.6 Comp.Ex. 34 T F, BF, M, γ 53.2 0.17 90 0 10 35 4.1 0.4 0.03 Example 35 T Experiment stopped due tobreaking of sheet by cold rolling Comp.Ex. 36 U F, BF, M, γ 4 2.7 0.1765 3 0 62 2.4 0.5 0.3 Example 37 V F, BF, 7 6 1.4 0.18 39 0 0 74 2.9 1.50.02 Example 38 W F, NRF, B, BF, 4 3.3 0.17 55 0 0 51 3.0 0.4 0.1Example tM, γ 39 X F, BF, M, γ 3 1.5 0.19 50 0 0 60 3.0 0.4 0.03 Example40 X F, BF, M, γ 3 6.1 0.26 43 0 35 73 8.1 0.8 0.02 Comp.Ex. 41 Y F, B,M, γ 3 2.4 0.28 82 0 0 51 2.0 0.3 0.03 Example 42 Z F, BF, M, γ 6 1.90.28 35 0 0 75 2.4 0.4 0.02 Example 43 Z F, BF, M, γ 5 9.1 0.33  0 0 4050 12.8  0.4 0.02 Comp.Ex. 55 AD F, BF, M, γ 1 2.1 0.23 51 0 0 60 1.71.5 0.02 Example 62 AI F, NRF, BF, M, γ 1 1.6 0.27 54 0 0 47 3.1 1.30.02 Example 65 AK BF, B, M, γ 2 1.4 0.19 31 0 0 63 4.7 2.3 0.03 Example76 AS F, BF, γ 13 1.3 0.28 37 0 0 50 0.5 0.7 0.1 Example 78 AT F, BF, γ8 1.6 0.35 56 0 0 46 3.1 2.3 0.01 Example 82 AW F, WF, BF, B, γ 4 1.90.31 53 0 0 49 2.8 1.3 0.02 Example 89 BC Experiment stopped due tocracking of slab during cooling Comp.Ex. 91 BE Experiment stopped due tocracking of slab during heating Comp.Ex. 92 BF Experiment stopped due tocracking of slab during heating Comp.Ex. 94 BH Experiment stopped due tocracking of slab during cooling Comp.Ex.

TABLE 12 Plated layer ζ Phase Ratio Base steel sheet of δ1Phase Aver-Aver- Bound- bound- Bound- age age ary ary ary thick- grainMicrostructure surface surface surface ness size Maxi- Experi- Chemi- γoccu- where occu- Plated of of mum mental cal Constitu- Frac- Contentpancy oxides pancy a- refined ferrite size of Exam- compo- tional tionFe Al ratio present ratio mount layer phase oxide ple nents structure %% % % % % g/m² μm μm μm Remarks C1 CA F, BF, 3 1.8 0.31 60 5 3 51 2.00.3 0.2  Example M, γ C2 CA NRF, F, 5 1.0 0.27 34 0 0 63 3.1 0.3 0.04Example B, γ C3 CA F, BF, B, 6 0.8 0.31 22 0 0 58 2.5 0.9 0.04 ExampleM, γ C4 CA F, M, θ 0 1.5 0.29 51 0 0 62 3.3 0.2 0.03 Comp.Ex. C5 CA F,P, B 0 1.8 0.30 59 5 0 54 3.5 0.3 0.2  Comp.Ex. C6 CA F, BF, γ 6 0.80.31 38 41 0 58 0.5 1.2 0.3  Example C7 CA F, B, BF, γ 4 0.6 0.74 25 0 060 3.5 0.4 0.1  Example C8 CA F, BF, γ 6 5.7 0.43 65 0 8 72 3.2 1.8 0.03Comp.Ex. C9 CA F, M 0 1.9 0.18 52 0 0 63 2.3 2.6 0.03 Comp.Ex. C10 CA F,BF, B, 2 6.1 0.33 48 0 12 64 3.5 2.3 0.03 Comp.Ex. M, γ C11 CB F, BF, B,γ 6 1.3 0.31 42 0 0 65 2.6 0.3 0.1  Example C12 CB F, BF, B, 3 1.8 0.1860 29 0 58 0.8 0.9 0.3  Example M, γ C13 CB F, BF, M, γ 1 1.6 0.23 50 130 53 1.5 0.4 0.3  Example C14 CB F, BF, B, γ 5 4.3 0.32 67 0 14 52 2.40.3 0.04 Example C15 CB F, P 0 1.6 0.35 57 0 0 60 2.1 0.4 0.03 Comp.Ex.C16 CB F, BF, M 0 1.7 0.27 54 0 0 56 1.8 0.4 0.02 Comp.Ex. C17 CB F, BF,B, 4 0.4 1.20 0 0 0 52 3.2 0.4 0.02 Comp.Ex. M, γ C18 CB Experimentstopped Comp.Ex. C19 CC F, BF, γ 9 2.3 0.20 68 0 5 45 1.5 0.3 0.1 Example C20 CC F, WF, BF, 6 1.5 0.17 45 0 0 65 1.7 0.3 0.1  Example M, γC21 CC F, B, tM, γ 11  2.0 0.30 57 0 0 72 1.3 0.6 0.04 Example C22 CCExperiment stopped Comp.Ex. C23 CC F, BF, M, γ 4 0.1 0.35 0 0 0 46 <0.1(3.3) (<0.01)   Comp.Ex. C24 CC F, M 0 2.4 0.20 51 0 0 54 1.7 2.3 0.01Comp.Ex. C25 CC F, P, BF, 0 2.1 0.28 75 0 0 58 1.8 1.8 0.04 Comp.Ex. B,M C26 CC F, BF, M 0 2.2 0.26 63 0 0 51 1.7 0.3 0.1  Comp.Ex. C27 CC BF,B, 10  1.8 0.34 60 0 0 50 1.7 0.4 0.1  Example M, γ C28 CD F, NRF, 0 1.20.21 37 0 0 57 1.7 1.3 0.03 Comp.Ex. θ, M C29 CD F, BF, M, γ 5 1.4 0.2734 8 0 67 1.9 0.3 0.04 Example C30 CD F, BF, B, M 0 1.5 0.26 36 0 0 632.3 1.4 0.04 Comp.Ex. C31 CD F, BF, B, 3 2.0 0.17 45 5 0 57 1.7 0.8 0.03Example M, γ C32 CD F, BF, B, 6 1.9 0.24 39 0 0 57 2.1 0.3 0.04 ExampletM, γ C33 CD F, BF, M, γ 5 0.4 0.82 22 0 0 49 1.4 1.5 0.03 Example C34CE F, B, BF 0 2.2 0.37 50 0 0 36 3.1 0.4 0.02 Comp.Ex. C35 CF F, B, BF,γ 2 1.3 0.23 65 0 0 46 3.1 0.4 0.03 Example C36 CF F, B, BF, γ 3 4.60.09 75 0 17 60 3.9 0.9 0.03 Example C37 CF F, B, BF, 0 1.2 0.20 60 0 050 2.5 0.4 0.04 Comp.Ex. P, θ C38 CG BF, M, γ 29  2.6 0.25 63 0 0 50 2.80.3 0.1  Comp.Ex. C39 CH F, θ 0 2.0 0.21 52 0 0 60 2.9 0.4 0.1  Comp.Ex.C40 CI F, P, θ 0 2.5 0.23 73 0 0 61 2.2 0.4 0.1  Comp.Ex. C41 CJ F, BF,M, γ 6 1.9 0.29 55 0 0 48 2.1 0.4 0.03 Comp.Ex. C42 CK F, BF, M, γ 6 2.00.22 51 0 0 55 2.2 0.4 0.02 Comp.Ex. C43 CL F, BF, B, 9 1.7 0.30 68 0 063 1.9 0.4 0.03 Comp.Ex. M, γ C44 CC F, BF, γ 8 2.2 0.21 72 73 14 50 2.30.7 0.2  Comp.Ex.

TABLE 13 Tensile properties Maximum Total Hole Experi- Chemi- tensileelonga- ex- Plat- Corro- Chip- Pow- mental cal Thick- strength tionpansibility ing Spot sion ping dering Exam- compo- ness t TS El λTS^(1.5) × El × adhe- weld- resis- Prop- prop- ple nents mm MPa % %λ^(0.5) sion ability tance erties erties Remarks 1 A 1.7 861 24 343.5E+06 ∘ ∘ ∘ ∘ ∘ Example 2 A 1.2 765 29 35 3.6E+06 ∘ ∘ ∘ ∘ ∘ Example 3A 1.8 694 35 29 3.4E+06 x ∘ ∘ ∘ ∘ Comp.Ex. 4 B 1.4 558 37 57 3.7E+06 ∘ ∘∘ ∘ ∘ Example 5 B 2.0 597 33 45 3.2E+06 ∘ ∘ ∘ ∘ ∘ Example 6 B Experimentstopped due to disablement of annealing treatment by shape defect ofsteel sheet Comp.Ex. 7 C 1.4 1017 21 29 3.7E+06 ∘ ∘ ∘ ∘ ∘ Example 9 C1.4 1010 18 28 3.1E+06 x ∘ x ∘ ∘ Comp.Ex. 10 D 1.3 738 27 37 3.3E+06 ∘ ∘∘ ∘ ∘ Example 12 D 2.3 853 23 41 3.7E+06 ∘ ∘ ∘ ∘ ∘ Example 13 E 13 80422 54 3.7E+06 ∘ ∘ ∘ ∘ ∘ Example 14 F 1.7 748 25 41 3.3E+06 ∘ ∘ ∘ ∘ ∘Example 15 G 1.2 741 31 34 3.6E+06 ∘ ∘ ∘ ∘ ∘ Example 17 I 1.4 950 23 283.6E+06 ∘ ∘ ∘ ∘ ∘ Example 18 J 1.9 1042 17 35 3.4E+06 ∘ ∘ ∘ ∘ ∘ Example20 J 1.0 981 17 46 3.5E+06 ∘ ∘ ∘ ∘ ∘ Example 21 K 1.3 1090 17 26 3.1E+06∘ ∘ ∘ ∘ ∘ Example 22 L 1.2 847 24 33 3.4E+06 ∘ ∘ ∘ ∘ ∘ Example 25 N 2.2948 22 27 3.3E+06 ∘ ∘ ∘ ∘ ∘ Example 26 N 1.0 885 23 25 3.0E+06 x ∘ ∘ x xComp.Ex. 27 O 2.0 918 22 38 3.8E+06 ∘ ∘ ∘ ∘ ∘ Example 28 P 1.8 582 27 532.8E+06 ∘ ∘ ∘ ∘ ∘ Example 29 Q 2.0 823 27 29 3.4E+06 ∘ ∘ ∘ ∘ ∘ Example30 R 1.3 804 26 30 3.2E+06 ∘ ∘ ∘ ∘ ∘ Example 31 S 1.2 755 26 35 3.2E+06∘ ∘ ∘ ∘ ∘ Example 33 S 1.0 692 27 27 2.6E+06 x ∘ ∘ ∘ x Comp.Ex. 34 T 1.4819 23 45 3.6E+06 ∘ ∘ ∘ ∘ ∘ Example 35 T Experiment stopped due tobreaking of sheet by cold rolling Comp.Ex. 36 U 1.4 1109 19 24 3.4E+06 ∘∘ ∘ ∘ ∘ Example 37 V 1.4 791 22 45 3.3E+06 ∘ ∘ ∘ ∘ ∘ Example 38 W 1.6692 26 38 2.9E+06 ∘ ∘ ∘ ∘ ∘ Example 39 X 1.7 639 32 49 3.6E+06 ∘ ∘ ∘ ∘ ∘Example 40 X 1.0 558 33 37 2.6E+06 x ∘ ∘ x x Comp.Ex. 41 Y 1.7 845 23 353.3E+06 ∘ ∘ ∘ ∘ ∘ Example 42 Z 1.9 1010 20 30 3.5E+06 ∘ ∘ ∘ ∘ ∘ Example43 Z 1.0 831 19 26 2.3E+06 ∘ ∘ ∘ x x Comp.Ex. 55 AD 1.0 643 29 423.1E+06 ∘ ∘ ∘ ∘ ∘ Example 62 AI 1.3 598 33 35 2.9E+06 ∘ ∘ ∘ ∘ ∘ Example65 AK 2.0 843 19 41 3.0E+06 ∘ ∘ ∘ ∘ ∘ Example 76 AS 1.4 781 28 353.6E+06 ∘ ∘ ∘ ∘ ∘ Example 78 AT 1.4 721 28 33 3.1E+06 ∘ ∘ ∘ ∘ ∘ Example82 AW 1.4 584 39 37 3.3E+06 ∘ ∘ ∘ ∘ ∘ Example 89 BC Experiment stoppeddue to occurrence of cracking of slab during cooling Comp.Ex. 91 BEExperiment stopped due to occurrence of cracking of slab during heatingComp.Ex. 92 BF Experiment stopped due to occurrence of cracking of slabduring heating Comp.Ex. 94 BH Experiment stopped due to occurrence ofcracking of slab during cooling Comp.Ex.

TABLE 14 Tensile properties Hole Experi- Chem- Maximum Total ex- Plat-Corro- Chip- Pow- mental ical Thick- tensile elongation pansibility ingSpot sion ping dering Exam- compo- ness t strength TS El λ TS^(1.5) × El× adhe- weld- resis- Prop- prop- ple nents mm MPa % % λ^(0.5) sionability tance erties erties Remarks C1 CA 1.7 861 24 34 3.5E+06 ∘ ∘ ∘ ∘∘ Example C2 CA 1.4 902 18 31 2.7E+06 ∘ ∘ ∘ ∘ ∘ Example C3 CA 1.2 765 2935 3.6E+06 ∘ ∘ ∘ ∘ ∘ Example C4 CA 1.2 733 18 26 1.8E+06 ∘ ∘ ∘ ∘ ∘Comp.Ex. C5 CA 1.2 587 23 41 2.1E+06 ∘ ∘ ∘ ∘ ∘ Comp.Ex. C6 CA 0.8 642 3742 3.9E+06 ∘ ∘ ∘ ∘ ∘ Example C7 CA 1.8 714 32 29 3.3E+06 ∘ ∘ ∘ ∘ ∘Example C8 CA 1.4 628 34 39 3.3E+06 x ∘ ∘ x x Comp.Ex. C9 CA 1.4 836 1924 2.2E+06 ∘ ∘ ∘ ∘ ∘ Comp.Ex. C10 CA 1.8 694 35 29 3.4E+06 x ∘ ∘ x xComp.Ex. C11 CB 0.7 558 37 57 3.7E+06 ∘ ∘ ∘ ∘ ∘ Example C12 CB 2.1 59930 43 2.9E+06 ∘ ∘ ∘ ∘ ∘ Example C13 CB 0.9 660 26 39 2.8E+06 ∘ ∘ ∘ ∘ ∘Example C14 CB 1.0 494 42 51 3.3E+06 ∘ ∘ ∘ ∘ ∘ Example C15 CB 1.2 567 1927 1.3E+06 ∘ ∘ ∘ ∘ ∘ Comp.Ex. C16 CB 1.2 701 15 28 1.5E+06 ∘ ∘ ∘ ∘ ∘Comp.Ex. C17 CB 1.0 637 28 41 2.9E+06 x ∘ ∘ ∘ ∘ Comp.Ex. C18 CBExperiment stopped Comp.Ex. C19 CC 1.8 1016 20 32 3.7E+06 ∘ ∘ ∘ ∘ ∘Example C20 CC 2.5 981 16 34 2.9E+06 ∘ ∘ ∘ ∘ ∘ Example C21 CC 1.8 930 2155 4.4E+06 ∘ ∘ ∘ ∘ ∘ Example C22 CC Experiment stopped Comp.Ex. C23 CC1.8 920 19 27 2.8E+06 x ∘ ∘ ∘ ∘ Comp.Ex. C24 CC 1.2 1033 9 19 1.3E+06 ∘∘ ∘ ∘ ∘ Comp.Ex. C25 CC 1.2 867 17 28 2.3E+06 ∘ ∘ ∘ ∘ ∘ Comp.Ex. C26 CC1.2 1068 13 25 2.3E+06 ∘ ∘ ∘ ∘ ∘ Comp.Ex. C27 CC 1.0 1201 11 42 3.0E+06∘ ∘ ∘ ∘ ∘ Example C28 CD 1.3 814 15 19 1.5E+06 ∘ ∘ ∘ ∘ ∘ Comp.Ex. C29 CD1.0 680 34 41 3.9E+06 ∘ ∘ ∘ ∘ ∘ Example C30 CD 1.4 666 26 29 2.4E+06 ∘ ∘∘ ∘ ∘ Comp.Ex. C31 CD 1.8 703 27 32 2.8E+06 ∘ ∘ ∘ ∘ ∘ Example C32 CD 2.2709 23 64 3.5E+06 ∘ ∘ ∘ ∘ ∘ Example C33 CD 0.8 649 31 29 2.8E+06 ∘ ∘ ∘ ∘∘ Example C34 CE 1.3 580 27 37 2.3E+06 ∘ ∘ ∘ ∘ ∘ Comp.Ex. C35 CF 1.4 62423 51 2.6E+06 ∘ ∘ ∘ ∘ ∘ Example C36 CF 1.6 604 28 45 2.8E+06 ∘ ∘ ∘ ∘ ∘Example C37 CF 1.4 510 26 37 1.8E+06 ∘ ∘ ∘ ∘ ∘ Comp.Ex. C38 CG 1.3 107527 19 4.1E+06 ∘ x ∘ ∘ ∘ Comp.Ex. C39 CH 1.3 362 32 95 2.1E+06 ∘ ∘ ∘ ∘ ∘Comp.Ex. C40 CI 1.0 449 25 52 1.7E+06 ∘ ∘ ∘ ∘ ∘ Comp.Ex. C41 CJ 1.2 67319 17 1.4E+06 ∘ ∘ ∘ ∘ ∘ Comp.Ex. C42 CK 1.7 740 14 19 1.2E+06 ∘ ∘ ∘ ∘ ∘Comp.Ex. C43 CL 1.9 905 10 8 7.7E+05 ∘ ∘ ∘ ∘ ∘ Comp.Ex. C44 CC 1.8 100020 35 3.7E+06 x ∘ ∘ ∘ ∘ Comp.Ex.

Experimental Example 89 is an example in which the experiment wasstopped since the Si content was high and the slab was cracked duringcooling in the casting step.

Experimental Example 92 is an example in which the experiment wasstopped since the P content was high and the slab was cracked duringheating in the hot rolling step.

Experimental Example 94 is an example in which the experiment wasstopped since the Al content was high and the slab was cracked duringcooling in the casting step.

Experimental Example 33 is an example in which since the heating rate inthe annealing step was low, the growth of the oxides in the base steelsheet excessively proceeded to form coarse oxides working as a fractureorigin on the surface of the base steel sheet, plating adhesion andpowdering properties were deteriorated.

Experimental Example C44 is an example in which since the preheatingcompletion temperature was high and a large number of coarse oxidesincluding Si and Mn were formed on the steel sheet surface beforeplating, the ratio of the interface formed between the grains in whichcoarse oxides are present and the base steel sheet with respect to theentire interface between the ζ phase and the base steel sheet was morethan 50%, and thus plating adhesion is inferior.

Experimental Example 9 is an example in which since the air ratio in theexcess heat zone in the annealing step was low, the boundary surfaceoccupancy ratio of the ζ phase was low, and unplating occurred in partsof the steel sheet, external appearance, plating adhesion, and corrosionresistance were deteriorated.

Experimental Example 43 is an example in which since the air ratio inthe excess heat zone in the annealing step was high and decarburizationin the steel sheet surface excessively proceeded, the average thicknessof the refined layer was thick and TS^(1.5)×El×λ^(0.5) was lowered sothat sufficient properties could not be obtained.

Experimental Example 3 is an example in which since the value ofExpression 1 was too small and a sufficient amount of the ζ phase wasnot formed at the interface between the plated layer and base steelsheet, and sufficient plating adhesion could not be obtained.

Experimental Example 26 is an example in which since the value ofExpression 1 in the plating step was too large and Fe % in the platedlayer was excessively high, sufficient plating adhesion could not beobtained.

Experimental Example C39 is an example in which since the C content waslow, residual austenite was not formed, and the volume fraction of thehard phase was low, sufficient tensile strength could not be obtained.

Experimental Example C38 is an example in which the C content was highand spot weldability and formability were deteriorated.

Experimental Example C40 is an example in which since the Mn content waslow, large amounts of pearlite and coarse cementite were formed in theannealing step and the plating step, and residual austenite was notformed, and the tensile strength and formability of the steel sheetcould not be sufficiently obtained.

Experimental Example 91 is an example in which the experiment wasstopped since the Mn content was high and the slab was cracked duringheating in the hot rolling step.

Experimental Example C41 is an example in which since the S content washigh and a large amount of coarse sulfides were formed, ductility andhole expansibility were deteriorated.

Experimental Example C42 is an example in which since the N content washigh and a large amount of coarse nitrides were formed, ductility andhole expansibility were deteriorated.

Experimental Example C43 is an example in which since the O content washigh and a large amount of coarse oxides were formed, ductility and holeexpansibility were deteriorated.

Experimental Example C34 is an example in which since Si and Al contentsdid not satisfy Expression (2), a large amount of carbides were formed,and residual austenite could not be obtained, a balance between strengthand formability was deteriorated.

Experimental Example C18 is an example in which the experiment wasstopped since the hot rolling completion temperature was low and theshape of the steel sheet was significantly deteriorated.

Experimental Example C22 is an example in which the experiment wasstopped since the temperature at which the steel sheet was coiled afterhot rolling was low and the steel sheet was broken in the cold rollingstep.

Experimental Example 6 is an example in which the hot-rolled steel sheetwas not subjected to cold rolling and the experiment was stopped sincethe degree of flatness of the sheet was poor and an annealing treatmentcould not be performed.

Experimental Example 35 is an example in which the experiment wasstopped since the rolling reduction in cold rolling was excessively highand the steel sheet was broken.

Experimental Example C4 is an example in which since the maximum heatingtemperature in the annealing step was low, residual austenite was notformed, a large amount of coarse cementite was present in the steelsheet, and TS^(1.5)×El×λ^(0.5) was deteriorated, sufficient propertiescould not be obtained.

Experimental Example C28 is an example in which since the maximumheating temperature in the annealing step was lower than Ac1+50° C.,residual austenite was not formed, a large amount of coarse cementitewas present in the steel sheet, and TS^(1.5)×El×λ^(0.5) wasdeteriorated, sufficient properties could not be obtained.

Experimental Example C23 is an example in which since the ratio betweenthe water vapor partial pressure P(H₂O) and the hydrogen partialpressure P(H₂), P(H₂O)/P(H₂), in the reduction zone in the annealingstep was low, the grain size of the surface was not refined, and ζ phaseformation did not proceed in the plated layer, plating adhesion wasdeteriorated. In Experimental Example C23, the refined layer was notformed, the average grain size of the ferrite in the surface of the basesteel sheet was 3.3 μm, and the maximum size of the oxides was less than0.01 μm inside the steel sheet within a range up to a depth of 0.5 μmfrom the surface.

Experimental Example 40 is an example in which since the ratio betweenthe water vapor partial pressure P(H₂O) and the hydrogen partialpressure P(H₂), P(H₂O)/P(H₂), in the reduction zone in the annealingstep was high, the refined layer of the surface of the base steel sheetwas excessively thick, and alloying of the plated layer excessivelyproceeded, plating adhesion, powdering properties, and chippingproperties were deteriorated.

Experimental Example C5 is an example in which since the average coolingrate from 750° C. to 700° C. was low, a large amount of carbides wasformed, residual austenite could not be obtained, a balance betweenstrength and formability was deteriorated.

Experimental Example C15 is an example in which since the averagecooling rate from 700° C. to 500° C. was low, a large amount of carbideswas formed, residual austenite could not be obtained, a balance betweenstrength and formability was deteriorated.

Experimental Example C24 is an example in which since the bainitictransformation treatment was not performed before or after the platingtreatment and residual austenite could not be obtained, a balancebetween strength and formability was deteriorated.

Experimental Example C25 is an example in which since the bainitictransformation treatment temperature before the plating treatment washigh, a large amount of carbides was formed, and residual austenitecould not be obtained, a balance between strength and formability wasdeteriorated.

Experimental Example C26 is an example in which since the bainitictransformation treatment temperature before the plating treatment waslow, the progress of bainitic transformation was excessively suppressed,and residual austenite could not be obtained, a balance between strengthand formability was deteriorated.

Experimental Example C9 is an example in which since the bainitictransformation treatment time before the plating treatment was short,bainitic transformation did not proceed sufficiently, and residualaustenite could not be obtained, a balance between strength andformability was deteriorated.

Experimental Example C37 is an example in which since the bainitictransformation treatment time before the plating treatment was long, alarge amount of carbides were formed, and residual austenite could notbe obtained, a balance between strength and formability wasdeteriorated.

Experimental Example C8 is an example in which since the bainitictransformation treatment temperature after the plating treatment washigh, the value of Expression (1) was excessively increased, and theamount of Fe in the plated layer was increased, plating adhesion wasdeteriorated.

Experimental Example C16 is an example in which since the bainitictransformation treatment temperature after the plating treatment waslow, the progress of bainitic transformation was excessively suppressed,and residual austenite could not be obtained, a balance between strengthand formability was deteriorated.

Experimental Example C30 is an example in which since the sum of thebainitic transformation treatment time before the plating treatment andthe bainitic transformation treatment time after the plating treatmentwas small, bainitic transformation did not sufficiently proceed, andresidual austenite could not be obtained, a balance between strength andformability was deteriorated.

Experimental Example C10 is an example in which since the amount ofeffective Al in the plating bath in the plating step was too small, thevalue of Expression 1 was too large, Fe % in the plated layer wasexcessively high, sufficient plating adhesion could not be obtained.

Experimental Example C17 is an example in which since the amount ofeffective Al in the plating bath in the plating step was excessivelylarge, the value of Expression 1 was excessively small, and a sufficientamount of the δ phase was not formed at the interface between the platedlayer, the base steel sheet and sufficient plating adhesion could not beobtained.

Experimental Examples other than the above examples are examples inwhich hot-dip galvanized steel sheets having excellent strength,ductility, hole expansibility, spot weldability, and plating adhesionwere obtained.

Example 2

A test piece was collected from the plated steel sheet of ExperimentalExample 1 obtained in “Example 1”. Next, the thickness cross section ofthe test piece parallel to the rolling direction of the base steel sheetwas used as an observed section and polished by ion milling and thus abackscattered electron (BSE) image was obtained with a field emissionscanning electron microscope (FE-SEM) under the condition of anaccelerating voltage of 5 kV. The results thereof are shown in FIG. 2.

As shown in FIG. 2, a plated layer including columnar grains formed ofthe ζ phase was formed in the plated steel sheet of ExperimentalExample 1. In addition, a refined layer in direct contact with theinterface with the plated layer was formed in the base steel sheet ofthe plated steel sheet of Experimental Example 1. As shown in FIG. 2,the refined layer of the plated steel sheet of Experimental Example 1included oxides (parts appearing darker than surroundings).

Example 3

A cold-rolled steel sheet was produced in the same manner as in theproduction of the plated steel sheet of Experimental Example 1 obtainedin “Example 1”, and subjected to an annealing step in the same manner asin the annealing of the plated steel sheet of Experimental Example 1 toobtain an annealed sheet. The annealed sheet was immersed in a zincplating bath under the conditions (the amount of effective Al, theplating bath temperature (bath temperature), the steel sheet enteringtemperature, the immersion time) for the plating step shown in Table 15to be plated.

After the plating step, a cooling treatment was performed under thecondition (Expression (1)) for the cooling step after plating shown inTable 15. Further, cold rolling was performed under the condition (therolling reduction) shown in Table 15 and thus plated steel sheets ofExperimental Examples 104 to 111 were obtained.

Regarding the obtained plated steel sheets, the plated layer of the basesteel sheet was observed in the same manner as in “Example 1”. Theresults are shown in Table 15.

Regarding the obtained plated steel sheets, the volume fraction (γfraction) of the residual austenite was measured in the same manner asin “Example 1”. The results are shown in Table 15.

Regarding the obtained plated steel sheets, the plated amount wasobtained in the same manner as in “Example 1”. The results are shown inTable 15.

Regarding the obtained plated steel sheets, the average thickness of therefined layer, the average grain size of the ferrite, and the maximumsize of the oxides were obtained in the same manner as in “Example F.The results are shown in Table 15.

Regarding the obtained plated steel sheets, a tensile test, a holeexpansion test, a bending test, an adhesion evaluation test, a spotwelding test, and a corrosion test were performed in the same manner asin “Example 1”. The results are shown in Table 15.

In addition, the results of Experimental Example 1 are collectivelyshown in Table 15.

TABLE 15 Base steel sheet Plated layer Aver- Aver- Plating step ζBound-ζ Oxides δBound- age age Plating bath Cooling Cold ary present aryMicro- thick- grain Effec- En- step rolling surface bound- surface Plat-structure ness size Maxi- Properties Experi- Chem- tive Bath teringImmer- after Rolling Fe Al occu- ary occu- ed γ of of mum Plat- Corro-mental ical Al temper- temper- sion plating reduc- con- con- pancysurface pancy a- Frac- refined ferrite size of ing Spot sion Exam-compo- % by ature ature time Expres- tion tent tent ratio ratio ratiomount tion layer phase oxide TS^(1.5) × adhe- welda- resis- ple nentsmass ° C. ° C. sec sion (1) % % % % % % g/m² % μm μm μm El × λ^(0.5)sion bility tance Remarks 1 A 0.104 459 459 3 0.62 0.16 1.7 0.34 62 0 061 3 3.1 0.8 0.03 3.5E+06 o o o Example 104 A 0.104 459 459 3 0.09 0.160.2 0.34  3 0 0 60 3 2.8 1.5 0.1 3.4E+06 x x o Comp.Ex 105 A 0.104 459459 10 1.11 0.2 1.2 0.31 50 0 0 64 3 3.4 1.4 0.1 3.4E+06 o o o Example106 A 0.104 459 459 20 1.70 0.2 2.4 0.30 68 0 0 70 3 3.4 1.1 0.1 3.4E+06o o o Example 107 A 0.104 459 459 70 2.11 0.2 4.1 0.32 100  0 0 80 2 4.00.9 0.1 3.4E+06 o o o Example 108 A 0.136 463 463 30 0.89 0.2 2.3 0.4370 0 0 70 3 2.4 1.2 0.04 3.4E+06 o o o Example 109 A 0.136 463 463 701.05 0.2 2.7 0.44 80 0 0 75 3 3.0 1.1 0.04 3.5E+06 o o o Example 110 A0.17 469 469 70 0.57 0.2 1.7 0.56 70 0 0 69 2 3.0 1.2 0.1 3.5E+06 o o oExample 111 A 0.17 469 469 100 0.82 0.2 1.8 0.56 100  0 0 75 3 2.5 1.50.1 3.5E+06 o o o Example

As shown in Table 15, Experimental Examples 105 to 111 as examples ofthe present invention had satisfactory plating adhesion and excellentspot weldability and corrosion resistance.

In contrast, as shown in Table 15, in Experimental Example 104, theratio of the interface between the ζ phase and the base steel sheet inthe entire interface between the plated layer and the base steel sheet(ζ boundary surface occupancy ratio) was less than 20% and thus platingadhesion and spot weldability were not sufficient.

Although each embodiment of the present invention has been described indetail above, all of these embodiments are merely examples ofembodiments in implementation of the present invention. The technicalscope of the present invention should not be interpreted as limited onlyby the embodiments. That is, the present invention can be implemented invarious forms without departing from the technical idea thereof or themain features thereof

INDUSTRIAL APPLICABILITY

The present invention is an effective technology for a hot-dipgalvanized steel sheet excellent in plating adhesion. According to thepresent invention, it is possible to provide a hot-dip galvanized steelsheet excellent in plating adhesion after forming.

1-5. (canceled)
 6. A hot-dip galvanized steel sheet comprising: a basesteel sheet; and a hot-dip galvanized layer formed on at least onesurface of the base steel sheet, wherein: the hot-dip galvanized layerincludes Fe in a content of more than 0% to 5% or less, Al in a contentof more than 0% to 1.0% or less, and columnar grains formed by a ζ phaseon the surface of the steel sheet, further, 20% or more of an entireinterface between the hot-dip galvanized layer and the base steel sheetis coated with the ζ phase, and a ratio of an interface formed between ζgrains in which coarse oxides are present among ζ grains and the basesteel sheet with respect to the entire interface between the ζ phase andthe base steel sheet in the hot-dip galvanized layer is 50% or less; thebase steel sheet includes a chemical composition which satisfies, % bymass, C: 0.040% to 0.400%, Si: 0.05% to 2.50%, Mn: 0.50% to 3.50%, P:0.0001% to 0.1000%, S: 0.0001% to 0.0100%, Al: 0.001% to 1.500%, N:0.0001% to 0.0100%, O: 0.0001% to 0.0100%, and Si+0.7Al≧0.30 (in theexpression, element symbols represent the content (% by mass) of eachelement), with a remainder of Fe and unavoidable impurities; the basesteel sheet has a refined layer in direct contact with the interfacebetween the base steel sheet and the hot-dip galvanized layer, anaverage thickness of the refined layer is 0.1 to 5.0 μm, an averagegrain size of ferrite in the refined layer is 0.1 to 3.0 μm, one or twoor more of oxides of Si and Mn are contained in the refined layer, and amaximum size of the oxide is 0.01 to 0.4 μm; and a volume fraction ofresidual austenite in a range of ⅛ thickness to ⅜ thickness centered ata position of ¼ thickness from the surface of the base steel sheet is 1%or more.
 7. The hot-dip galvanized steel sheet according to claim 6,wherein a plated amount on one surface of the base steel sheet in thehot-dip galvanized layer is 10 g/m² or more and 100 g/m² or less.
 8. Thehot-dip galvanized steel sheet according to claim 6, wherein the basesteel sheet further contains, % by mass, one or two or more selectedfrom Ti: 0.001% to 0.150%, Nb: 0.001% to 0.100%, and V: 0.001% to0.300%.
 9. The hot-dip galvanized steel sheet according to claim 7,wherein the base steel sheet further contains, % by mass, one or two ormore selected from Ti: 0.001% to 0.150%, Nb: 0.001% to 0.100%, and V:0.001% to 0.300%.
 10. The hot-dip galvanized steel sheet according toclaim 6, wherein the base steel sheet further contains, % by mass, oneor two or more selected from Cr: 0.01% to 2.00%, Ni: 0.01% to 2.00%, Cu:0.01% to 2.00%, Mo: 0.01% to 2.00%, B: 0.0001% to 0.0100%, and W: 0.01%to 2.00%.
 11. The hot-dip galvanized steel sheet according to claim 7,wherein the base steel sheet further contains, % by mass, one or two ormore selected from Cr: 0.01% to 2.00%, Ni: 0.01% to 2.00%, Cu: 0.01% to2.00%, Mo: 0.01% to 2.00%, B: 0.0001% to 0.0100%, and W: 0.01% to 2.00%.12. The hot-dip galvanized steel sheet according to claim 8, wherein thebase steel sheet further contains, % by mass, one or two or moreselected from Cr: 0.01% to 2.00%, Ni: 0.01% to 2.00%, Cu: 0.01% to2.00%, Mo: 0.01% to 2.00%, B: 0.0001% to 0.0100%, and W: 0.01% to 2.00%.13. The hot-dip galvanized steel sheet according to claim 9, wherein thebase steel sheet further contains, % by mass, one or two or moreselected from Cr: 0.01% to 2.00%, Ni: 0.01% to 2.00%, Cu: 0.01% to2.00%, Mo: 0.01% to 2.00%, B: 0.0001% to 0.0100%, and W: 0.01% to 2.00%.14. The hot-dip galvanized steel sheet according to claim 6, wherein thebase steel sheet further contains, % by mass, one or two or moreselected from Ca, Ce, Mg, Zr, La, and REM in a total amount of 0.0001%to 0.0100%.
 15. The hot-dip galvanized steel sheet according to claim 7,wherein the base steel sheet further contains, % by mass, one or two ormore selected from Ca, Ce, Mg, Zr, La, and REM in a total amount of0.0001% to 0.0100%.
 16. The hot-dip galvanized steel sheet according toclaim 8, wherein the base steel sheet further contains, % by mass, oneor two or more selected from Ca, Ce, Mg, Zr, La, and REM in a totalamount of 0.0001% to 0.0100%.
 17. The hot-dip galvanized steel sheetaccording to claim 9, wherein the base steel sheet further contains, %by mass, one or two or more selected from Ca, Ce, Mg, Zr, La, and REM ina total amount of 0.0001% to 0.0100%.
 18. The hot-dip galvanized steelsheet according to claim 10, wherein the base steel sheet furthercontains, % by mass, one or two or more selected from Ca, Ce, Mg, Zr,La, and REM in a total amount of 0.0001% to 0.0100%.
 19. The hot-dipgalvanized steel sheet according to claim 11, wherein the base steelsheet further contains, % by mass, one or two or more selected from Ca,Ce, Mg, Zr, La, and REM in a total amount of 0.0001% to 0.0100%.
 20. Thehot-dip galvanized steel sheet according to claim 12, wherein the basesteel sheet further contains, % by mass, one or two or more selectedfrom Ca, Ce, Mg, Zr, La, and REM in a total amount of 0.0001% to0.0100%.
 21. The hot-dip galvanized steel sheet according to claim 13,wherein the base steel sheet further contains, % by mass, one or two ormore selected from Ca, Ce, Mg, Zr, La, and REM in a total amount of0.0001% to 0.0100%.